Methods For Forcing A Liquid Into A Low Pressure Formation

Abstract

A method for driving a well fluid column indicating a liquid chemical agent into a low-natural pressure formation containing oil and gas, through the use of a lightweight foam as the driving fluid so that, after this driving, the low pressure within the formation will be sufficient to force the foam out of the well thereby allowing the oil and gas production of the well to resume naturally. This method is applicable for the stimulation of wells having a low-natural pressure formation.

Description

FIELD OF THE INVENTION

This invention relates generally to the production of oil and gas from low-pressure oil and gas bearing earth formations and relates more specifically to work-over operations on low-pressure oil and gas wells. The invention is specifically directed to a new method for injecting or driving well fluid back into low-pressure earth formations as well as to derived methods concerning more particularly the stimulation of wells which would have been unproductive after a conventional stimulation operation because of insufficient formation pressure.

BACKGROUND OF THE INVENTION

The injection of various fluids into oil and gas bearing formations in order to improve the production of a well extending to the formation is a stimulation method well-known to those skilled in the art. When the formation pressure is higher than the hydrostatic pressure generated by the driving fluid present in the well, there is no problem in immediately restoring the production of the well after releasing the pressure of the driving fluid, simply because the higher formation pressure simply ejects the column of fluid from the well and production of well fluid begins. That conventional stimulation method is not applicable without killing the well when the formation pressure is lower than the hydrostatic pressure generated by the driving fluid present in the bore-hole.

Likewise, when the formation pressure is relatively low and the well flows only as a result of the presence of dissolved gas or gas in bubbles in the oil, a problem arises whenever the production of the well is stopped and the oil has time to degasify. In this case, the gas gathers at the top of the well while the level of the increasingly degasified oil drops gradually. If, at that moment, the well is opened, the gas escapes and the well is unproductive owing to insufficient formation pressure, that is to say, the hydrostatic pressure of a degasified oil column having a height lower than the depth of the well is then balancing or overbalancing the formation pressure, depending upon the permeability of the formation. The recompletion of wells which are unproductive owing to insufficient formation pressure is so difficult and costly that, in certain regions, it may be more desirable to continue production and burn the produced oil and gas rather than cease production if it is at all possible to do so.

One method for recompleting a well which is unproductive owing to insufficient formation pressure consists of reinjecting the degasified oil into the formations by means of a driving fluid consisting of a neutral gas (natural gas or liquefied nitrogen) injected under pressure into the well, the use of air being ruled out because of the risk of explosion. This method has certain drawbacks, in particular that of being costly and of requiring relatively cumbersome liquid gas tanks. Moreover, owing to the very low hydrostatic pressure generated by the gas, it requires relatively high injection pressure with all the attendant safety problems involved.

Accordingly, it is a primary object of the present application to provide a novel method for stimulating and treating low-pressure oil and gas producing wells wherein a light-weight compressible medium is injected into the well under sufficient pressure to drive the well fluid column standing in the well back into the formation where it will regasify, thereby enabling production to begin naturally as soon as the pressure on the well is released.

It is also an important object of the present invention to provide a novel method of injecting a compressible light-weight medium into a well wherein the compressible medium is generated through the use of low-cost materials and through the use of low-cost injection apparatus.

Another feature of the present invention is the provision of a novel method for stimulating and treating low-pressure oil and gas wells wherein stimulation operations may be accomplished readily and simply without creating the hazards that are attendant other methods of well stimulation.

It is also a feature of the present invention to provide a novel method for stimulating and treating low-pressure oil and gas wells, which method is of simple nature, is reliable in use and low in cost.

Other and further objects, advantages and features of the present invention will become apparent to one skilled in the art upon consideration of the written specification, the attached claims and the annexed drawings. The form of the invention, which will now be described in detail, illustrates the general principles of the invention, but it is to be understood that this detailed description is not to be taken as limiting the scope of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a novel method for stimulating and treating low-pressure oil and gas wells which may conveniently take the form of injecting a light-weight driving fluid into such wells under sufficient pressure to force at least a substantial portion of a column of liquid formed by a mixture of a chemical agent and crude oil standing in the well back into the formation and closing the well at the surface where the column of light-weight fluid will prevent production fluid from rising within the well. After the well has been closed for a desirable period of time and resumption of production is desired, the pressure at the well head will be released and the formation pressure, being sufficient to raise the column of light-weight fluid, will eject the light-weight fluid and liquid chemical agent from the well and oil and gas will then be produced in normal manner by the pressure of the formation. The light-weight fluid may take the form of a foam fluid that is mixed at the well site through the use of simple foam mixing apparatus. Gelatin liquid separators may be utilized if desired to separate the foam and/or the crude oil from any of the chemical agents.

BRIEF DESCRIPTION OF THE DRAWING

So that the manner in which the above recited features, advantages and objects of the present invention, as well as others which become apparent, are attained and can be understood in detail, more particular description of the invention, briefly described above, may be had by reference to the embodiment thereof which is illustrated in the appended drawing, which drawing forms a part of this specification.

It is to be noted however, that the appended drawing illustrates only a typical embodiment of the invention and is, therefore not to be considered as limiting of its scope, for the invention may admit to other equally effective embodiments.

IN THE DRAWING:

The unigue FIGURE is a pictorial schematic representation of an oil and gas well extending to an oil bearing earth formation and having a tank of liquid chemical agent and a foam generating apparatus both connected to the well head of the well and arranged for injection of chemical agent and foam into the well.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A compressible, low-density medium such as foam (emulsion of a gas - air for example - in a high-surface tension liquid) has been used as a circulation fluid either for the drilling of wells or for the cleanout of well debris. It will be noted that in the case of an air foam, there is no danger of explosion because the air is then in the form of small bubbles trapped in liquid film which forms the continuous phase of the emulsion. The use of foam as a circulation fluid has been described in different publications and more particularly in the U.S. Pat. Nos. 3,463,231 and 3,410,344.

Up to the present time, foam has not been proposed as a driving fluid for the stimulation of wells in low-pressure earth formations. The present invention relates precisely to the conditions for the use of foam in such well stimulation operations.

The equipment necessary for foam production is well known to the prior art and is exemplified schematically in the FIGURE. It consists primarily of two alternately employed tanks 10 and 12 containing a high-surface-tension liquid (sweet or salt water to which is added a surfactant - 1 to 2 percent in general - or even oil) in which tanks are immersed two conduits connected by a change-over valve 14 at the inlet of a foam compressor or pump 16 which can, for examle, by means of a gearbox 17 and a motor 19, generate two constant volume flow rates respectively V.sub.1 and V.sub.2 dm.sup.3 /min, with a maximum output pressure of P.sub.l kg/cm.sup.2. It also includes an air compressor 18 with a variable delivery (for example from U.sub.1 to v.sub.2 m.sup.3 /min) also having a maximum output pressure of P.sub.g kg/cm.sup.2. In the case where the natural gas (or any other gas, and in particular nitrogen) in liquid form or under pressure is available in the well, the use of such a compressor is not necessary. In addition to the pump and the compressor, foam generating equipment comprises a mixing chamber 20 of elongated cylindrical form one end of which has two inlet pipes 22 and 24 and the other end one outlet pipe 26. This chamber is filled with slightly compressed stainless metal chips intended to ensure the effective mixing of the liquid and of the gas respectively delivered by the pump and the compressor to the two inlet pipes so as to produce a good quality foam.

A given foam is characterized by its liquid mass concentration .phi.. In other words, if water is injected into the mixing chamber of a foam generator, to which is added a surfactant with a mass flow of M kg/min and air with a mass flow of m kg/min, there is obtained, at the output, a foam with a liquid mass concentration .phi. = M/ (M+m). By way of example, for M = 200 kg/min and m = 26 kg/min (or a volume flow rate of 20 m.sup.3 /min at atmospheric pressure), there is obtained a foam with a concentration .phi. = 0.885 and a mass flow rate D.sub.m = 226 kg/min. By varying the relative mass flow rates of the surfactant-containing liquid and of the gas, stable foams may be obtained (i.e., conserving their physical properties for at least 10 hours or so) from a liquid mass concentration .phi. of about 0.7 up to .phi. = 0.98 which, for air and water, corresponds to volume ratios of 330 and 16 respectively under ambient conditions.

When it is desired to treat a well as 27 with chemicals to stimulate production or to accomplish any other desirable result using foam as the driving fluid to force the chemicals to the desired location within the well or formation, the following procedure is used. After ceasing production of the well and shortly thereafter (before degasifying of the oil) a suitable volume of the treating chemicals may be injected from a tank 28, through a pump 30 and a pipe 32 into the well (a few cubic meters for example), so that the same volume of oil is consequently reinjected into the formation 33. Instead of continuing to pump chemicals, degasified oil or water, all of which are fluids that, owing to their high densities, may cause the well to become unproductive due to insufficient formation pressure, foam is utilized as the injection medium. The paratmeters of the foam to be injected (concentration and amount of foam) are then determined according to the particular characteristics of the well. When all the liquids (chemicals and crude oil) initially contained in the well have been injected or reinjected into the formations under the force of the foam, an interval sufficient to allow the chemicals to act is then observed. After this interval, the valve of the well is opened and the natural recompletion conditions of the well are met, since the formation pressure is then higher than the hydrostatic pressure generated by the foam column.

Another utilisation of the method according to the invention is also of value. It concerns the application of a corrosion inhibiting or descaling product, depending on the case, to the metallic casing of the well. Such inhibiting products are liquid chemical compositions well known to the art. As the density of these liquids is relatively high, in order to prevent them from falling to the bottom of the well in the form of drips of varying size, during a first phase, there may be introduced into the well, by pumping, a separating plug made up of a certain amount of gelatin which, owing to its very high viscosity, remains at the surface of all liquids (whatever their density). During a second phase, the volume of inhibiting product considered necessary in accordance with the height of the well to be treated may be pumped into the well. During a third phase, a second gelatin plug may be introduced into the well by pumping.

These three phases typically take no longer than an hour or two, i.e. a time sufficiently short so that the oil present in the well does not have the time to degasify significantly. At the end of the said third stage, pumping of foam may be begun. The liquid mass concentration and the amount of the foam to be injected into the well will be determined again from the particular characteristics of the well. When the first gelatin plug has been lowered to the convenient depth, under the action of the foam injected into the well, it is sufficient to stop this injection and to open the head valve in order for the well to begin producing immediately.

Owing to the process according to the invention the chemical composition is regularly applied as a coating upon the casing wall. Besides during the injection then the ejection of the foam into and from the well, such a coating is not eroded because of the low viscosity of said foam.

Another procedure would consist, preferably after having introduced a gelatin plug into the well, of pouring into the well a sufficient volume of water to generate a make-up hydrostatic pressure which would proportionately relieve the injection pressure then required from the foam generator, provided however that the final hydrostatic pressure generated at the reservoir formation level is lower than the formation pressure.

It is seen, therefore, that the present invention is one well adapted to attain all of the objects and advantages hereinabove set forth, together with other advantages which will become obvious and inherent from a description of the apparatus itself. It will be understood that certain combinations and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the present invention.

As many possible embodiments may be made of this invention without departing from the spirit and scope hereof, it is to be understood that all matters hereinabove set forth or shown in the accompanying drawings are to be interpreted as illustrative and not in any limiting sense.

Claims

We claim:

1. In a process for treating wells extending from the surface of the earth to a reservoir formation from which oil having a gas in solution therein is produced, the steps of:

injecting into the well a sufficient volume of a liquid chemical agent adapted to stimulate said reservoir formation;

injecting a sufficient column of stable foam into the well under sufficient pressure to drive the column of liquids then present into the well back into the reservoir formation;

terminating pressure injection of said foam into said well when at least a substantial portion of said column of liquids has been forced back into said formation;

closing said well at the surface for a desired period of time; and

opening the well and allowing said foam column and eventually the remaining portion of said column of liquids to rise normally under the influence of formation pressure, thereby ejecting said foam as a column formed by a mixture of liquid chemical agent and of oil and gas in solution rises within said well.

2. The process of claim 1, wherein the volume of foam that is injected into the well is sufficient to force all of the liquids within the well back into the formation without injection of any of the foam into the formation.

3. The process of claim 1, wherein said well is maintained closed for a sufficient period of time to enable the liquid chemical agent forced into the reservoir formation to increase the formation permeability by a convenient coefficient.

4. The process of claim 1, including the method steps of injecting a quantity of gelatin into said well prior to said injection of foam.

5. The process of claim 4 including the further step of injecting a quantity of water into said well prior to said injection of foam.

6. In a process for treating a well casing with a liquid chemical agent, which casing extends from the surface of the earth to a formation from which an oil/gas solution is produced, and wherein the pressure of the formation is insufficient to raise a column of degasified oil, the steps of:

injecting a quantity of gelatin into said well to prevent any substantial mixing of the oil and gas solution with the chemical agent;

injecting a desired quantity of said chemical agent into said well;

injecting a second quantity of gelatin into said well to prevent any substantial mixing of said chemical agent and a foam;

injecting a sufficient column of stable foam into the well under sufficient pressure to drive the column of oil standing in said well back into said formation, the hydrostatic pressure developed at the level of the formation by the column of oil/gas solution, gelatin, chemical agent and foam being less than the formation pressure;

terminating pressure injection of said foam into said well when the second quantity of gelatin has reached a convenient depth in the well;

releasing the pressure at the well head of the well and allowing the column of foam, chemical agent, gelating and oil gas solution within said well to rise under the natural influence of formation pressure.