On the Issue of Cleaning Formation Fluid from Sand and Gas

ON THE PURIFICATION OF FORMATION FLUID FROM SAND AND GAS

L. A. Pukhlyakov

(Presented by Professor A. V. Aksarin)

Submersible pumps, both sucker-rod and rodless (electric centrifugal), by means of which oil and water are extracted from wells, require protection from the harmful influence of sand and gas. The presence of sand in the rising fluid causes premature wear of individual parts of submersible pumps, and sometimes leads to their jamming; the presence of gas prevents the timely opening of the valves of sucker-rod pumps, which causes a reduction in their productivity. The productivity of electric centrifugal pumps is reduced in a similar manner. Gas-sand anchors have found wide application for purifying the rising fluid from sand and gas.

One of the most widespread types of gas-sand anchors is the YaGP2-4 gas-sand anchor, consisting of gas and sand chambers connected by a coupling [3, pp. 362–363]. Through the inlet openings of such an anchor the formation fluid rises through the annular space and first enters the gas chamber, where gas bubbles are released from it. Then, through the inlet tube, it is directed downward into the sand chamber, where it changes the direction of motion to the opposite one and, through the inner tube, is directed to the pump intake. In the process of turning, individual grains of sand, as bodies having a specific gravity greater than that of water or oil, rush downward into the zone of lower mobility of the fluid and fall onto the bottom of the sand chamber.

The main disadvantage of this gas-sand anchor is that its sand chamber is single-section. Because of this, its cross section proves to be much smaller in comparison with the cross section of the well. Hence the velocity of motion of the fluid inside it proves to be greater than during the motion of this fluid from the bottomhole to the bottom of the anchor. This, in turn, means that the grains of sand rising in the given fluid stream, for the most part, should not settle in the anchor—which is what actually occurs. In fact, the type of gas-sand anchor described above is not a satisfactory means for protecting the pump from the harmful influence of sand.

In order for sand to settle in sufficient measure in the anchor, the velocity of the rising stream inside it must be made many times smaller than its velocity in the wellbore between the bottomhole and the bottom of the anchor. This can be achieved by making the anchor in the form of a multi-section assembly on the principle of a multi-section bailer for tubeless drilling [1, pp. 124–127]. An attempt to create such an anchor was undertaken by O. A. Sidorov and L. S. Melik-Aslanov, who called their invention a gas-sand separator [2].

The Sidorov and Melik-Aslanov gas-sand separator is a long pipe on which a large number of settling chambers are fitted. When a downhole pump is equipped with this separator, the formation fluid rises through the annular space and is gradually drawn off into the individual chambers. Here its velocity decreases sharply, since the individual chambers are connected to the pipe in parallel. As a result, the formation fluid is cleaned of sand better than in the YAGP2-4 gas-sand anchor.

The Sidorov and Melik-Aslanov gas-sand separator did not become widely used, and one of the reasons for this is the following shortcoming, inherent in many known types of sand anchors. The fluid rises to the openings of this anchor through the annular space, and this does not allow a tailpipe to be attached to it (a small-diameter pipe usually attached to the pump intake to improve removal of sand from the bottomhole of the well). The absence of a tailpipe is accompanied by intensive accumulation of sand at the bottomhole of the well, which leads to clogging of the filter, i.e., the perforated part of the casing string, and to cessation of inflow into the well. This phenomenon is called plug formation, and combating it is a rather difficult problem. To remove plugs from the well, the production equipment is lifted, after which the sand plug is either bailed out with a bailer or washed out with a jet pump. These operations involve large material costs and loss of oil production. Therefore field operators very often choose to replace downhole pumps more frequently, but not to allow sand plugs to form at well bottomholes. Otherwise they remove the sand anchors from the pump intakes and attach tailpipes to them. Thus, we arrive at the conclusion that it is necessary to create such a sand anchor that would not preclude the use of a tailpipe. Such an anchor must be of the two-pipe type. In this case, through one pipe (the supply pipe) the fluid must be delivered to the cleaning chambers, and through the second (the discharge pipe) it must be led away from them. Such an anchor has been developed and, at the suggestion of employees of the Committee for Inventions and Discoveries under the Council of Ministers of the USSR, has been named a downhole gas-sand separator.

The two-pipe downhole gas-sand separator consists of a large number of sections, each of which, in turn, consists of a settling chamber 1 and two small pipes passing through it: a supply pipe 2 and a discharge pipe 3 (Fig. 1). As the formation fluid moves through the supply pipe, its flow must be distributed uniformly among all the settling chambers. For this purpose, inlet small pipes 4 are used. With their wide ends these small pipes are welded to the supply pipes, while their narrow ends are directed downward (toward the bottoms of the settling chambers). Individual lengths of the discharge pipe are provided with several discharge openings 5. The lowest of these openings are located above the inlet small pipes.

Individual lengths of the discharge and supply pipes are provided at their ends with flanges 6, with which they form a single whole (connected by threading). By means of threading they are also connected to the bottom of the settling chamber. The settling chambers are closed with covers 7, each of which consists of two halves joined by strips. This allows the covers to be moved along the pipes. To clean the settling chambers of the rock that has settled in them, these chambers are provided with windows 8, which are closed by special screw-in shutters. The individual sections, or links, of the separator are connected to one another by bolts 9 and nuts 10 in such a way that, after connection, the individual lengths of the supply and discharge pipes form unified supply and discharge pipes.

In the upper part of the separator there is a gas chamber 11, pre-

Fig. 1. Two-pipe downhole gas-sand separator (basic diagram): 1 — settling chamber, 2 — supply tube, 3 — discharge tube, 4 — inlet tube, 5 — discharge openings, 6 — flanges, 7 — covers of the settling chambers, 8 — windows for removing rock from the settling chambers, closed by screw-in shutters, 9 — connecting bolts, 10 — nuts of the connecting bolts, 11 — gas chamber, 12 — tube for accumulating gas released from the oil as it moves along the supply tube, 13 — valve, 14 — outlet, 15 — curved branch pipe for directing oil to the side wall of the gas chamber, 16 — central tube, 17 — gas-discharge branch pipe, 18 — valve, 19 — upper connecting coupling, and 20 — lower connecting coupling.

constituting a mechanism for the final purification of the liquid from gas. Through it passes the continuation of the feed tube, which is a mechanism for preliminary purification of the liquid from gas, consisting of a tube for gas accumulation 12, a valve 13, and a branch 14.

The very upper link of the discharge tube, called the curved branch pipe 15, ends inside the gas chamber. Through it, the oil purified of sand enters the gas chamber in such a way that it first moves horizontally and then descends along the wall to the bottom of the gas chamber. The lower parts of the gas-accumulation tube 12 and the curved branch pipe 15 are welded to a flange with two holes for bolts, by means of which the lower separator links are fastened to the gas chamber.

For removing liquid from the gas chamber there is a central tube 16, the lower end of which is located near the bottom of the gas chamber. For removing gas from the gas chamber, it is provided with a gas-discharge branch pipe 17 with a valve 18.

In the upper part of the gas chamber there is a connecting coupling 19, by means of which the separator is attached to the intake of the pump. In the lower part of the very lowest link of the separator there is a second connecting coupling 20, which serves for attaching a tailpipe to the separator.

The operation of the two-tube downhole gas-sand separator consists in the following. By means of the pump, formation fluid is drawn out of the gas chamber and, consequently, also out of the discharge tube. This, in turn, causes its inflow into this tube through openings in the upper parts of the individual sections from the settling chambers. The liquid enters the chambers only through the inlet tubes, which are directed downward, and to them through the feed tube. Thus, the stream of liquid saturated with sand first rises as a common mass through feed tube 2, and then is divided into several parts (according to the number of sections) and, through the narrowed cross sections of inlet tubes 4, enters settling chambers 1. Here the liquid stream expands; hence its velocity decreases, and the liquid begins slowly to rise toward the openings of the discharge tubes. In the course of this rise the liquid will settle—individual grains of sand and a considerable quantity of clay particles will precipitate from it.

In the discharge tube, the liquid rising upward will again acquire considerable velocity, but this will already be liquid purified of sand and of a substantial portion of clay particles, which will not cause premature wear of individual parts of the downhole pump.

On leaving discharge tube 3 through curved branch pipe 15 into gas chamber 11, the liquid will strike the wall of this chamber at high velocity and move downward along a spiral line. Owing to this, centrifugal forces will arise in the liquid, promoting better separation of gas from it. This gas, having accumulated in a large quantity, will open valve 18 and, through gas-discharge branch pipe 17, escape into the annular (casing) space. Having descended to the lower part of gas chamber 11, the oil will be drawn from it through central tube 16.

The operation of the mechanism for preliminary purification of the oil from gas consists in the following. Separating from the oil in the process of its rise through feed tube 2, the gas must accumulate in tube 12 under valve 13. When its quantity becomes considerable, it will open the valve and, through branch 14, will exit into the annular (casing) space.

References

1. L. A. Pukhlyakov. One of the problems of cable-tool drilling and the possibility of its solution. Proceedings of the Tomsk Polytechnic Institute, vol. 166, 1969.

2. O. A. Sidorov, L. S. Melik-Aslanov. Gas-sand separator. Bulletin of Inventions, No. 16, 1961.

3. R. D. Faniev. Operation of Oil and Gas Fields. Gostoptekhizdat, 1958.