ON INCREASING CORE RECOVERY IN OIL EXPLORATION EXPEDITIONS OF TOMSK OBLAST

Yu. L. Boyarko, D. A. Ucharov

(Presented by the scientific seminar of the Department of Exploration Technology)

Increased core recovery in exploratory drilling of oil and gas wells is one of the serious and as yet unresolved problems facing subsurface explorers in Tomsk Oblast. From the rock samples brought up from the drilled intervals, one can judge the presence of oil or gas, their quantity, the reservoir properties of the strata, etc. In this connection it would be desirable for the core to be brought up in full from the entire horizon of interest to geologists. But at the present level of technological development this is impossible, since part of the core is washed away, abraded, and so forth. Therefore drillers are assigned the task of extracting no less than 60% of the core from the drilled interval. But in a number of cases drillers cannot cope even with this task—the core barrel is brought to the surface empty or almost empty, and this sharply reduces the overall percentage of core recovery for the well.

The Department of Exploration Technology of Tomsk Polytechnic Institute, having joined in the work of assisting the Tomsk oil workers, carried out a certain amount of research on this topic. A comprehensive study of the question raised made it possible to draw the following preliminary conclusions: low core recovery depends both on poor technical equipment and on incorrect drilling technology, with the latter factor having the greater effect.

From the technical point of view, there is a lack of proper matching of core bits with the corresponding core barrels. Thus, in a number of cases, when SDK 1 drill heads are used, KTD3 core barrels are employed, while the body of the core bit is an ordinary thick-walled drill pipe. Such an assembly leads to skewing and vibration of the core barrel in the body and, as a consequence, to a sharp reduction in core recovery.

However, violations of the technological procedure are much more numerous. The mismatch between the type of drill head and the rock being drilled, the mismatch of the core catchers used, and nonobservance of the drilling-technology regimes in the interval of core taking—all this is the main reason for low core recovery.

Let us turn to the distribution diagram of runs with different percentages of core recovery shown in Fig. 1, constructed from materials of 90 runs made for the purpose of extracting core in a number of wells of the Aleksandrovsk expedition. The presence of two maxima on the diagram is immediately noticeable. One of them falls on that part of the runs in which

in which the recovery is from 0 to 10% of the core, and the other—from 50 to 60%. If the second maximum indicates the possibility, at the present level of technology, of obtaining the planned percentage of core recovery, then the first maximum eloquently shows that there are a large number of causes preventing the core from entering the core barrel or preventing it from being retained in the core barrel during lifting. These principal causes are mainly violations of the drilling technology within the core-taking interval.

Fig. 1. Distribution diagram of runs with different percentages of core recovery for the Aleksandrovskaya expedition

As is known, when drilling with a core bit it is recommended to use a special regime that provides for a deliberately reduced mechanical rate of penetration in order to increase the percentage of core recovery. Under this regime, the bit, located several meters above the bottom, is set in rotation and drilling fluid begins to be pumped through it. This is done so that, during the slow approach to the bottom, all the cuttings that have settled on it are lifted and do not fall onto the core barrel at the very beginning of the run. Naturally, all these operations take a considerable amount of time, but this fact is taken into account when standards are drawn up.

However, observations have shown that in many cases the specified special drilling regime is not observed, either because the physical meaning of the coring process is not understood, or because of a desire to increase the rate of penetration of the well; the result is an excessively rapid placement of the tool on bottom and a sharp increase in pressure on the bit. The core barrel becomes clogged with loose fractions of the rock, and with further rotation this rock is smeared in the section of the core lifter, which interferes with the filling of the core tube.

It is interesting to note the following fact. In well R 202, in the core-taking interval at a depth of 1000 m, during testing of a new core lifter, one of the drillers was asked to try to recover the maximum amount of core using a core lifter of the old design, which was being challenged by the new one. With all parameters of the special regime observed, 100% of the core was recovered; moreover, the rock in the interval drilled was represented by weathered, water-saturated sandstone, for which 20–30% core recovery is considered a quite good operating result.

Another less noticeable, but quite important, reason for the failure of the core barrel to fill with core is the careless attitude of the drilling

cuttings to the parts and assemblies of the core bit. Before being assembled for a new run, the core barrel is often not inspected, cleaned, or lubricated. This leads to poor operation of the core catcher and the drainage valve and, accordingly, to loss of core. Meanwhile, the thickness of the drilled interval is constantly increasing, and empty runs sharply dilute the overall indicators of the percentage of core recovery, although, as can be seen from the diagram, almost half of all runs have fairly good quantitative indicators (from 50 to 100% core recovery).

More than 30% of all the runs studied are empty and almost empty runs (from 0 to 20%). And, finally, 20% of the runs were made with core recovery from 20 to 50%. We explain such a low percentage of core recovery by the mismatch between the types of core bits and the rocks being penetrated. As a rule, a well has one type of drill head and core catcher, while the rocks, in their lithological and physicomechanical composition, change rather often; this naturally leads to the fact that for some horizons we obtain an increased percentage of core recovery, and for others a low one.

The analysis carried out makes it possible to conclude that, at present, the main line of work for increasing the percentage of core recovery should consist not in designing new bits—although this, too, must be pursued—but in developing in drilling crews the ability to apply, when necessary, special drilling regimes competently and with full responsibility.

The specific proposals in this case amount to the following: for drillers, it is necessary systematically to organize refresher courses, with the corresponding lectures delivered at a high theoretical level. When drilling in intervals requiring core sampling, control at drilling rigs over compliance with the drilling regime prescribed by the instructions should be strengthened. A material incentive should be introduced for drilling crews by paying bonuses for increasing the percentage of core recovery. Further research should be continued on the development and introduction of core bits suitable for the drilling conditions in Tomsk Oblast.