Issues of Methodology and Techniques for Testing Oil and Gas Wells in Tomsk Oblast

G. A. Podobin

(Presented by Professor A. V. Aksarin)

Well testing is the most important and most responsible stage of exploratory drilling. The fate of the discovery of new oil and gas fields, and sometimes of oil-and-gas-bearing regions, depends on the results of well testing, just as it does on the correct determination of the locations for drilling wells and on the selection of the entire set of geological prospecting operations.

In recent years, 7 gas and 15 oil fields have been discovered within the oblast.

However, these successes do not diminish but rather increase the role of well testing. Testing is assigned the most important task: to obtain, in a timely manner, exhaustive data on productive strata, necessary for calculating oil and gas reserves and for preparing field-development projects. In connection with major discoveries, the volume of work is continuously increasing (Table 1). Therefore, shortening the duration of testing is becoming extremely important.

Thus, testing is becoming one of the decisive factors determining not only the geological but also the economic efficiency of exploratory drilling.

Nevertheless, despite this, testing still remains the greatest bottleneck in the overall complex of prospecting and exploration work for oil and gas: testing times are still very long, and the methodology and techniques for carrying out the work are insufficiently refined.

The dynamics of the time balance of the well-construction cycle, given in Table 1, indicate that, despite the decrease in the number of tested horizons in a well, the share of time spent on testing is continuously increasing. Over the past 6 years, the ratio of testing time to drilling time has increased threefold.

Analysis shows that even if the existing shortcomings in the organization, planning, and technical equipment of the work are eliminated, the time spent on testing will nevertheless, in the coming years, exceed the time spent drilling wells.

Consequently, in order to eliminate the existing gap in the rates of productivity growth in testing and drilling, it is necessary not only to improve substantially the organization of testing operations, but also to introduce fundamental changes into their methodology.

The currently existing methodology for testing productive strata in prospecting and exploration wells after pene-

…the entire productive interval and cementing it with production casing (the so-called “bottom-up” system) has a number of substantial disadvantages.

The main ones are as follows:

1. When negative results are obtained in the course of testing, the costs associated with cementing the well with production casing may prove unjustified.

Table 1

1. Often, obtaining an inflow of oil or gas from an underlying bed delays for a long time the testing of overlying beds.

2. Because of the reduction in the natural permeability of beds in the near-wellbore zone during drilling penetration and cementing of the casing string, it is not always possible to reliably assess potential flow-rate capabilities without applying methods of inflow intensification.

3. Testing beds in a cased well is greatly prolonged because of the numerous perforations of the casing string, the installation of bridge plugs, poor-quality cementing, and also prolonged drainage of beds with a contaminated near-wellbore zone.

These shortcomings hinder the further development not only of well testing, but also of the entire complex of petroleum exploration operations.

At present, the most progressive direction, ensuring a significant increase in the efficiency of geological exploration for oil and gas, is the testing of wells while drilling, using formation testers.

Abroad, this approach to conducting geological exploration has become fundamental; the wide use of formation testers has made it possible to substantially shorten the time and reduce the cost of petroleum exploration operations.

This has been achieved above all because, with the aid of formation testers, it became possible to combine drilling and testing into a single technological process.

G. S. Gazyan [2], processing data on well drilling for five major oil regions of the United States and Canada, showed that for all regions and depths it is characteristic that a “dry” exploration well is 30% cheaper than a production well, while the times required for drilling and investigating them are approximately the same. At the same time, in our country an exploration well costs 2–3 times more than a production well, and the time required to drill it is 2–3 times greater.

In the United States, various modifications of formation testers (FT) have been created and are widely used; they are run into the well on drill pipes and on cable, and a theoretical basis has also been developed and computing centers created for interpreting the data, which make it possible, in a short time, to carry out the entire set of operations for formation testing and to obtain its full hydrodynamic characterization.

In our country, the development of FT designs began considerably later than in the United States; therefore, naturally, for a long time domestic FTs were insufficiently advanced and allowed only trial testing to be carried out, i.e., inducing inflow and obtaining a qualitative characterization of the formation.

This is, in the main, what explains the slow pace of their introduction.

In recent years, GrozNII and UfNII have developed, and at present domestic industry is serially producing, sets of testing tools of the GrozUfNII Design and Research Institute, which, under favorable geological and technical conditions, make it possible to obtain the necessary minimum of quantitative data on the formation.

Along with the development of new FT designs, the methodology for interpreting the data obtained with their aid has also been improved.

A whole series of methods quite acceptable for practical application has been proposed for processing inflow curves and bottomhole-pressure buildup curves, making it possible, with sufficient accuracy, to evaluate the parameters of the formation and its deliverability potential [1, 5, 6].

Thus, at present the scale and effectiveness of applying the new method of testing wells during drilling with the aid of FTs depend mainly on the geological structure of the region and the conditions under which wells are drilled.

The geological features of the section that determine the methodology and technology of well trial testing during drilling include the following:

1. A great thickness and heterogeneity of the oil-and-gas-bearing stage.
2. Lithological discontinuity of productive beds in thickness and along strike (horizons of groups Yu, A, PK).
3. Frequent alternation of sandy-clayey varieties, both lithologically and in terms of the nature of saturation with fluids.
4. An insufficient number of distinct geophysical markers.
5. Discrepancy of structural maps for Jurassic and Cretaceous deposits.
6. Difficulty in interpreting field-geophysical data because of the clayiness of beds in Cretaceous deposits and the presence of dense shielding beds in Jurassic deposits.

Under such conditions, high geological effectiveness can be obtained with the aid of FTs.

However, the listed features of the section, as well as the insufficient degree of study of the region, substantially complicate correlation of the section and the selection of intervals for core sampling, and, consequently, the selection of objects for trial testing during drilling.

At the first stage of work on introducing the method of testing wells in an uncased borehole using a formation tester (FT), the following principal tasks were set.

1. Determination of the geological and technical conditions and selection of an arrangement of the testing tools that would ensure lowering of the FT with a “full-size” packer to the planned depth in working condition and leak-tight packer setting.

2. Determination of the hydrodynamic conditions and the maximum possible time of formation drainage ensuring normal operation of the FT, unsetting of the packer, and lifting of the testing tools without complications.

3. Evaluation of the reliability of the test results.

The first experimental work on testing formations with an FT in an uncased borehole was carried out in wells completed by drilling.

These were, for the most part, wells drilled to the basement and not penetrating horizons with clearly oil-bearing or gas-bearing characteristics.

As a rule, formations were selected for FT testing from which inflows or indications of oil and gas had been obtained on adjacent areas.

Testing was carried out “from bottom to top,” with separating cement bridges set between the intervals.

In the course of the experimental work, in addition to the technical issues mentioned above, one of the principal tasks of testing formations with an FT in an uncased borehole was also addressed: reducing the cost of “dry” exploratory and appraisal wells by eliminating expenditures on casing the borehole with production casing.

Initially, an arrangement of testing tools based on the GrozNII formation tester with a disk valve was used (Fig. 1).

Subsequently, various arrangements of the standard set of testing tools of the KII GrozufNII-146 were used (Fig. 2a). The principal criterion in assessing the quality of testing was the recovery of formation fluid.

If clay mud or its filtrate was recovered from the formation, repeated FT runs were made until formation fluid was obtained; in the absence of inflow, control runs were made.

Even the first experimental work showed that, when testing in an uncased borehole in wells completed by drilling (“from bottom to top”), far from all the capabilities of the formation tester are utilized.

As is known, one of the principal advantages of using an FT is that the formation can be tested during drilling, immediately after penetration, when it is still only slightly contaminated and the permeability of the near-wellbore zone is close to natural. This ensures the acquisition of reliable information on formation productivity, which is decisive in the search for oil and gas. When a well is tested after drilling has been completed, the formations are maximally contaminated. For this reason, first, in order to obtain formation fluid, repeated FT runs have to be made; second, assessment of the formation is greatly hampered; and third, because of the formation of caverns and constrictions, passage of packers through the borehole and packer setting become complicated, which entails additional expenditures of money and time for drilling out destroyed rubber packer elements after unsuccessful runs, repeated well reaming, and FT runs.

For example, when testing the low-permeability formation A-1 in well No. 20 of the Sovetskoe field after drilling was completed, 9 FT runs were required to obtain a geological result, of which

Fig. 1. a—arrangement of testing tools based on the GrozUFNII IP; b—rod-suspension device; c—unit for installing a cast-iron diaphragm. 1—shoe, 2—tailpiece, 3—filter, 4—paired packer, 5—equalizing valve; 6—poppet valve, 7—locking device, 8—cast-iron diaphragm, 9—square rod, 10—stud; 11—cable, 12, 14—adapter, 13—rod

Fig. 2. Arrangement of the GrozUFNII formation tester: a—with paired packer. 1—shoe, 2—tailpiece, 3—filter, 4,7—downhole pressure gauge, 5—paired packer, 6—1st GrozUFNII assembly, 8—circulation valve, 9—drill pipes; b—with double packering. 1—shoe, 2,5—filter, 3,8—downhole pressure gauge, 4—equalizing valve, 6—packer, 7—1st GrozUFNII formation-tester assembly, 9—clay mud, 10—cast-iron diaphragm, 11—drill pipes, 12—locking device

of which only two proved successful. During the second run of the formation tester, together with the filtrate of the clay mud, a small amount of oil was recovered. In well 7 of the Severo-Mezhovskaya area, when testing the Barabinskian sequence, characterized by low reservoir properties, only during the second run of the formation tester, together with the filtrate of the clay mud, was a small amount of formation water obtained; at the same time, 4 m³ of clay-mud filtrate was recovered from the formation.

A major shortcoming of testing wells with a formation tester “from the bottom upward” is the need to set isolation bridges. In the best case, setting a cement bridge takes 2 days. However, in the course of testing, at the moment the intake valve of the formation tester is opened, cement bridges are often torn away; as a result, all the work of preparing the well for testing has to be repeated.

In addition, unsuccessful setting of bridges is sometimes accompanied by substantial expenditures of funds and time to eliminate complications caused by cement entering the clay mud. All this sharply reduces the effectiveness of using the formation tester.

The examples cited clearly illustrate contamination of the near-wellbore zone of formations that remain for a long time under the influence of a column of clay mud, and also confirm the well-known proposition that the negative effect of clay mud is manifested to the greatest extent in low-permeability formations.

The results of the work carried out on testing formations with a formation tester in an uncased borehole after completion of drilling (“from the bottom upward”) give grounds to believe that this method is insufficiently effective and cannot be recommended for broad implementation.

It is acceptable only in exceptional cases, for example when it is necessary to test a formation missed during drilling, or a projected horizon.

The accumulated experience of working with formation testers made it possible to proceed to testing wells during drilling. The very first formation tests during drilling in wells No. 38 and No. 37 at the Sovetskoye field confirmed the advantages of the “top-down” testing method.

In these wells horizon A-1 was tested; in both cases oil was obtained in one run of the formation tester.

As already mentioned above, because of the frequent alternation of permeable beds and the unfavorable position of geophysical benchmarks, in a number of cases—especially in new areas—it is difficult to identify the beds to be tested in a timely manner without a significant increase in the volume of field-geophysical investigations and core sampling.

As a result, by the time of testing, not only the formation subject to testing but also the underlying one often turns out to have been penetrated.

In such a case it becomes necessary to isolate the formation with packers at the top and bottom.

For these purposes, an arrangement of testing tools with a double packer was used (Fig. 2b).

The use of double packering proved very effective in interval testing of productive horizons for the purpose of studying the nature of formation saturation over thickness and determining the position of the water-oil contact (WOC).

The use of double packering makes it possible to penetrate the WOC at once and then carry out the required number of tests.

The basic information on testing wells with formation testers is given in Table 2.

In 20 wells, tests were carried out in the casing string on intervals tested by the formation tester during drilling.

In all cases, the data on the qualitative characteristics of the formations obtained with the aid of IP were confirmed.

The scale and effectiveness of testing during drilling depend in many respects on the volume and reliability of the quantitative data on formations obtained with the aid of IP. Unfortunately, the testing tools are still not equipped with reliable downhole instruments that provide recording of pressure and buildup curves.

Table 2

However, recently simple devices have been proposed which, in most cases, ensure the recording of pressures at the bottomhole by means of standard downhole pressure gauges. In five wells, inflow curves and curves of change in bottomhole pressure were recorded and processed; as a result, acceptable quantitative data were obtained.

As already mentioned, at the first stage of introducing IP, exploratory wells were tested in which, on the basis of core and field-geophysical data, no clear indications of oil-and-gas content had been obtained.

Naturally, most of these wells were not cased after IP testing.

As a result, in addition to solving geological problems, savings were achieved in casing pipe and cement, and the time required to complete the well was considerably reduced by eliminating casing and testing in the casing string.

Subsequently, in connection with the increase in the number of discovered oil and gas fields, the geological effectiveness of the work rose sharply. Thus, in 1966 the efficiency coefficient was 0.5, and in 1967 it was 0.8.

In this connection, doubts began to be expressed as to the expediency of using IP in fields.

At present, the high efficiency of testing wells with IP during drilling has been confirmed not only in exploratory drilling, but also in the process of detailed appraisal.

We shall show this using the example of the Sovetskoye oil field and the Myldzhinskoye gas-condensate field.

The Sovetskoye oil field is one of the largest in Western Siberia.

The structure is complicated by five local uplifts. Productive horizon A-I is traced in all the uplifts. Approximately 70% of the pool is water-floating; bottom water is absent only in the uplift located in the central part of the field.

According to core and field-geophysical data, the OWC is not clearly distinguished.

As a result of testing a number of wells on the central uplift and the areas adjacent to it, the OWC zone was determined within the interval 1650–1654 m (here and below, absolute elevations).

When drilling other uplifts, in many wells from reservoir A1, when it was perforated above the assumed OWC, inflows of oil with water or water with an oil film were obtained. Moreover, in a number of watered wells the satisfactory quality of cementing was confirmed.

It became necessary to study the saturation and productivity of reservoir A1 over its thickness.

To solve this problem under the given geological conditions, the most reliable method proved to be interval-by-interval testing of the IP horizon during its penetration by drilling.

During 1966–1967, interval-by-interval testing of horizon A1 by IP was carried out at the field in 19 wells. In some wells, 4–6 intervals were tested. The total number of tests was 54.

As a result of this work it was established that in different parts of the pool, depending on the reservoir properties of the bed, the interval of the OWC zone varies within much wider limits than had previously been assumed.

For example, in well 80, using IP, the position of the OWC was determined at a depth of 1630.4 m; in well 56, at 1644.2 m.

The data obtained during IP testing of reservoir A1 will greatly facilitate interpretation of geological and geophysical materials and, ultimately, will make it possible to refine the oil reserves in the pool and the technological scheme for its development.

At the Myldzhinskoye gas-condensate field, four gas-bearing horizons have been identified: Yu-II, Yu-I (Jurassic), and B-XVI–XX, B-VIII (Lower Cretaceous). The B-XVI–XX, B-VIII, and Yu-II reservoirs are gas-saturated in individual areas of the field. Interpretation of electric logging data is extremely difficult because of the clayey nature of the reservoirs and the presence of shielding interbeds.

On the basis of core and geophysical data, it is not possible to determine with confidence the character of reservoir saturation. For verification purposes, all reservoirs had to be tested in casing. As a result, in a number of wells, after obtaining a gas flow from reservoir YuI, water was obtained from reservoirs B-XVI–XX and B-VIII.

For example, in wells 32 and 29 the geophysical characteristics of reservoir B-XVI–XX do not differ substantially, but from the first well a gas flow was obtained with a rate of 30 thousand m³/day, while from the second there was an inflow of water of 0.9 m³/day.

As is known, at present there is a regulation according to which every exploratory well that has yielded commercial inflows of oil or gas must be conserved for transfer to the main stock.

Therefore, wells in which, after obtaining a gas flow from overlying horizons, water was obtained are subject to repair or to writing off under category III. The low effectiveness of repair work is known, and such wells are often written off.

Testing of potentially productive horizons during drilling made it possible to study more thoroughly the gas-bearing character of the section, to reduce the number of tests in casing, and to conserve, for transfer to the main stock, all wells from which commercial gas inflows were obtained.

Testing by IP during drilling is of great importance for assessing the oil-and-gas-bearing potential of Upper Cretaceous deposits.

To evaluate them after the well has been cemented with the production casing, it is necessary to ensure a cement rise of about 2000 m and to significantly increase the number of intervals tested in the casing. Ensuring the necessary height of cement rise under the conditions of Western Siberia—because of the impossibility of concentrating a sufficient amount of equipment at the drilling site for cementing and because of the possible absorption of cement slurry—is associated with great difficulties, while increasing the number of intervals tested in the casing considerably prolongs the testing process.

By testing the well with a formation tester (IP) during drilling, these difficulties are eliminated. In this case the effectiveness of the new methodology is obvious.

Thus, in Tomsk Oblast the following tasks can be solved with the aid of formation testers:

1. Study of the oil-and-gas-bearing capacity of the section.

This will make it possible, first, on the basis of the industrial evaluation of formations obtained during drilling, to adjust the further direction of exploration work; second, during detailed exploration of the main productive horizon, for the purpose of estimating reserves in categories A and B, to evaluate the reserves of other formations in lower categories; third, to test a larger number of formations and thereby reduce the probability that oil-and-gas-bearing horizons will remain undetected; fourth, to reduce the time required for testing in casing, to save casing pipe and cement, and to shorten the time spent completing “dry” wells by eliminating their casing and testing in the casing.

1. Study of the oil-and-gas saturation of formations over their thickness.

This will make it possible to refine the position of the WOC, the reserves of pools, and the technological schemes for their development.

1. Selection of objects for investigations in the casing.

This will make it possible to eliminate unnecessary expenditures of time on testing unproductive horizons in the casing and to increase the number of exploratory wells suitable for transfer to the main producing stock.

Conclusions

1. The geological and economic effectiveness of oil-prospecting work depends largely on the results of well testing. To improve the quality and productivity of testing, it is necessary not only to improve the organization of work, but also to substantially change its methodology.

2. In Tomsk Oblast, where most oil and gas fields are multilayered, testing wells with an IP during drilling is highly effective not only in prospecting and exploration drilling, but also in detailed exploration.

In the near future this testing method should become the basic one in geological exploration for oil and gas.

1. Testing formations with an IP during drilling, under normal conditions of well drilling, does not present substantial technical difficulties.

2. The geological and technical conditions of the region make it possible, with the aid of an IP, to obtain a quantitative characterization of formations.

References

1. Borisov Yu. P., Mukharskii E. D. Determination of certain parameters of oil reservoirs with the aid of formation testers. Neftyanoe khozyaistvo, No. 1, 1966.

1. G. S. Gazyan. Fundamentals of a rational methodology for opening and testing formations in the course of drilling an exploratory well. Geologiya nefti i gaza, No. 1, 1962.

2. P. S. Lapshin. Determination of formation parameters by means of UFNII test instruments. Neftyanoe khozyaistvo, No. 7, 1963.

3. K. I. Loshkarëv, S. S. Repin. Experience in the use of the GrozNII formation tester. Neftyanoe khozyaistvo, No. 7, 1963.

4. E. B. Chekalyuk. A universal method for determining the physical parameters of a formation from measurements of bottomhole pressures and inflows. Neftyanoe khozyaistvo, No. 2, 1964.

5. E. B. Chekalyuk. Determination of formation parameters from the results of well testing using a tester. Scientific and Technical Collection on Oil Production. Issue 19. Gostekhizdat, Moscow, 1963.