1. Field of the Invention
The present invention relates generally to the field of hydrocarbon production from hydrocarbon-bearing formations. More particularly, it concerns fluid compositions that can be useful in improving hydrocarbon production and methods of introducing such fluid compositions into a hydrocarbon-bearing formation.
2. Description of Related Art
Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (e.g., a xe2x80x9creservoirxe2x80x9d) by drilling a well that penetrates the hydrocarbon-bearing formation. This provides a partial flowpath for the oil to reach the surface. In order for oil to be xe2x80x9cproduced,xe2x80x9d that is travel from the formation to the well bore (and ultimately to the surface), there must be a sufficiently unimpeded flowpath from the formation to the well bore. Unobstructed flow through the formation rock (e.g., sandstone, carbonates) is possible when rock pores of sufficient size and number are present for the oil to move through the formation.
A common reason for a decline in oil production is xe2x80x9cdamagexe2x80x9d to the formation that plugs the rock pores and impedes the flow of oil. Often such damage is a result of methods and chemicals used in establishing the well or in remedial operations performed on it. Another reason for lower than expected production is that the formation is naturally xe2x80x9ctightxe2x80x9d (e.g., a low permeability formation), with pores sufficiently small that the oil migrates toward the well bore only very slowly.
In general, techniques used to increase the permeability of the formation are referred to as xe2x80x9cstimulation.xe2x80x9d Essentially, one can perform a stimulation technique by: (1) injecting chemicals into the well bore to react with and/or dissolve damage; (2) injecting chemicals through the well bore and into the formation to react with and/or dissolve small portions of the formation to create alternative flowpaths for the hydrocarbon (thus rather than removing the damage, redirecting the migrating oil around the damage); or (3) injecting chemicals through the well bore and into the formation at pressures sufficient to fracture the formation, thereby creating a channel through which hydrocarbon can more readily flow from the formation and into the well bore.
Acids or acid-based fluids, and fluids containing strong chelants have been found to be useful both for the removal of damage and the creation of alternative flowpaths due to their ability to dissolve certain formation minerals and/or certain contaminants introduced into the well bore and formation during drilling or subsequent operations. The most common agents used in acid treatments of wells are mineral acids, though organic acids are also used. Hydrochloric acid is the preferred acid treatment in carbonate formations. For sandstone formations, the preferred fluid is a hydrochloric/hydrofluoric acid mixture. When the treatment is designed to remove damage resulting from a drilling mud filter cake, it is important to moderate the reaction of the fluid with the filter cake. When a strong acid is delivered in a low-viscosity fluid, often the action of removing the low permeability filter cake is localized, and much of the remainder of the acid is lost to the formation without further removal of the filter cake across the damaged zone. Often drilling mud is designed with a combination of polymers and dissolvable minerals, such as calcium carbonate. An emulsion formulated with an internal acid or internal strong chelant can be used to remove the mineral on a moderated basis by slow release. Dissolution of the mineral compromises cake strength making it more easily removed by production pressures. Acids are also utilized in the process called xe2x80x9cacid fracturingxe2x80x9d for improving permeability in carbonate reservoirs through an etching technique.
At present, such acid-based and chelant-based treatments have several serious limitations: lack of uniform radial and axial (along the wellbore) coverage; corrosion of the pumping equipment and well bore tubing; and reaction rates at higher temperatures that are too rapid. An improved well stimulation fluid that has a moderated reactivity and that is less damaging to equipment is a highly desirable goal.
Hydraulic fracturing involves literally breaking or fracturing a portion of the surrounding strata, by injecting a specialized fluid into the well bore directed at the face of the geologic formation at pressures sufficient to initiate and extend a fracture in the formation. Actually, what is created by this process is not always a single fracture, but a fracture zone, that is, a zone in the formation having multiple fractures, through which hydrocarbon can more easily flow to the well bore.
Certain commonly used fracturing treatments generally comprise at least three principal components: a carrier fluid (usually water or brine), a polymer, and a proppant. Many further comprise a crosslinker. Other compositions used as fracturing fluids include water with additives, viscoelastic surfactant gels, and gelled oils. The purpose of these fracturing fluids is to first create and extend a fracture, and then once it is opened sufficiently, to deliver proppant into the fracture, which keeps the fracture from closing once the pumping operation is completed. The carrier fluid is the means by which proppant and breaker are carried into the formation. A typical fracturing fluid can be prepared by blending a polymer with an aqueous solution (sometimes an oil-based or a multi-phase fluid is desirable); often, the polymer is a solvatable polysaccharide. The purpose of the polymer is to increase the viscosity of the fracturing fluid which aids in the creation.of a fracture; and to thicken the aqueous solution so that solid particles of proppant can be suspended in the solution for delivery into the fracture. In many fracturing treatments, a crosslinking agent is added which further increases the viscosity of the fluid composition by crosslinking the polymer.
In addition to being useful in fracturing, fluids that comprise a polymer and crosslinker can also be useful in the workover of a hydrocarbon producing well to improve production. Such workover fluids (e.g., those used in conformance control and zone abandonment operations, among others) with increased viscosity through polymer crosslinking, especially those that become a gel after crosslinking, are useful for isolating particular zones for subsequent treatment by acids, scale removers, or asphaltene diluents, among others. After the treatment, a gel formed by the workover fluid can be intentionally degraded to remove the barrier. In other cases, the gel permanently plugs the zone.
A chemical or biological agent (e.g., crosslinked polymer, acid, or biocide, among others) that is a useful component of a stimulation or workover fluid can, in certain cases, be neutralized or degraded before reaching the site at which it is to have its effect. Therefore, in certain instances, more of the agent is used in order to be effective and to compensate for agent that is lost in delivering the agent to the site. Thus, there is a need for a more efficient way to deliver useful chemical and biological agents to a desired location in a well.
The present invention is directed to fluid compositions that can be used in improving hydrocarbon production and methods of introducing such fluid compositions into a hydrocarbon-bearing formation. The invention relates to delaying the action of at least one chemical or biological agent that is sequestered in the discontinuous phase of an emulsion. Upon exposure to one or more destabilizing conditions, the emulsion is disrupted, releasing the sequestered chemical or biological agent into the bulk fluid of the composition, permitting the agent to have its desired effect.
One embodiment is a method of delivering a well treatment fluid composition into a hydrocarbon-bearing formation. In this method, a well treatment fluid that comprises a surfactant stabilized emulsion is pumped into a well. The emulsion comprises a continuous phase and at least one discontinuous phase that comprises at least one chemical or biological agent. The emulsion is destabilized by exposure to at least one of (1) a change in temperature, (2) a change in pH, (3) a change in salinity, (4) a change in alcohol concentration, (5) a change in stabilizing surfactant concentration, (6) a change in organic ion concentration, (7) a change in destabilizing surfactant concentration, (8) a change in surfactant adsorbent material concentration, (9) an ultrasonic pulse, and (10) an electrical field. Destabilization of the emulsion results in the release of the chemical or biological agent from the discontinuous phase.
Another embodiment is directed to a method of delivering a well treatment fluid composition, as described above, into a hydrocarbon-bearing formation. In this embodiment, the well treatment fluid is pumped into a well that comprises a first zone and a second zone. The first zone has a first ambient condition, and the second has a second ambient condition that is different from the first ambient condition in at least one respect (e.g., temperature, pH). The fluid composition comes into contact with the first zone, thereby exposing the fluid composition to the first ambient condition, and later the fluid composition comes into contact with the second zone, exposing it to the second ambient condition. The emulsion of the well treatment fluid remains stable for at least about one minute when the fluid is in contact with the ambient condition in the first zone. However contact with the ambient condition in the second zone destabilizes the emulsion, thereby releasing the chemical or biological agent. The ambient conditions of the first zone and the second zone differ in at least one of (1) temperature, (2) pH, (3) salinity, (4) alcohol concentration, (5) stabilizing surfactant concentration, (6) organic ion concentration, (7) destabilizing surfactant concentration, (8) surfactant adsorbent material concentration, (9) ultrasonic energy, and (10) an electrical field.
Yet another embodiment is directed to a method of delivering a well treatment fluid composition into a hydrocarbon-bearing formation, in which, prior to pumping the fluid into the well, at least one chemical that undergoes a chemical reaction or that dissolves in the well treatment fluid composition is added and mixed into the composition. As the chemical reaction of the chemical proceeds or as it dissolves, it causes a change in at least one of the following characteristics of the well treatment fluid: (1) pH, (2) alcohol concentration, (3) stabilizing surfactant concentration, (4) organic ion concentration, (5) salinity, and (6) destabilizing surfactant concentration. The change in the fluid that results is sufficient to destabilize the emulsion, thus causing the chemical or biological agent to be released. Naturally the pumping of this well treatment fluid must occur before the reaction of the chemical or dissolution of the chemical goes to completion; if this were not the case the release of the chemical or biological agent would not be sufficiently delayed.
Still another embodiment is directed to a well treatment fluid composition, comprising a surfactant stabilized emulsion, wherein the emulsion comprises a continuous phase and at least one discontinuous phase, the discontinuous phase comprising at least one chemical or biological agent. The emulsion in the fluid composition will destabilize sufficiently to release the chemical or biological agent upon exposure to at least one of the conditions listed above. Well treatment fluid compositions of the present invention can be used, for example, in stimulation and workover of oil, gas and water wells, and thus these compositions can be fracturing fluids, acidizing treatment fluids, zone abandonment fluids, conformance control fluids or drilling mud filter cake cleanup fluids depending on their composition and the manner of their application. Well treatment-fluids of the present invention can also be biocide treatment fluids, corrosion inhibition fluids, or breaker fluids.
It is preferred that the emulsion of the fluid compositions of the present invention be a microemulsion, because microemulsions tend to have greater stability than macroemulsions. Preferably, the chemical or biological agent initially in the discontinuous phase is selected from the group consisting of crosslinkers, polymers, biocides, corrosion inhibitors, corrosion dissolvers, pH modifiers, breakers, metal chelators, metal complexors, antioxidants, wetting agents, polymer stabilizers, clay stabilizers, scale inhibitors, scale dissolvers, wax inhibitors, wax dissolvers, asphaltene precipitation inhibitors, waterflow inhibitors, sand consolidation chemicals permeability modifiers, foaming agents, microorganisms, nutrients for microorganisms, and salts. Fluid compositions and methods of the present invention can be used for improved fracturing or workover operations by permitting a delay of crosslinking. For example, a preferred embodiment of the present invention is a fluid composition that comprises a cross-linkable polymer in the continuous phase of the emulsion and a crosslinker in the discontinuous phase of the emulsion. Eventually, the crosslinker is released into the continuous phase due to exposure of the emulsion to an emulsion destabilizing effect. Thus, the crosslinking of the polymer is delayed until the emulsion is destabilized. Alternatively, instead of sequestering the crosslinker to delay crosslinking, the polymer or a pH modifier (the pH modifier causes the pH of the fluid composition to change to a pH favorable for the crosslinking reaction) can be sequestered in the discontinuous phase of an emulsion until the emulsion is disrupted. Thus, in certain embodiments, it is preferred that the polymer and the crosslinker can react with each other readily when the emulsion is destabilized. Such fluid compositions comprising a polymer that becomes crosslinked are particularly useful as fracturing fluids, but can also be used in workover operations.
The delay in crosslinking afforded by certain embodiments of the present invention is beneficial in that the amount of energy required to pump the fluids can be reduced, the penetration of certain fluids can be improved, and shear and frictional damage to polymers can be reduced. By delaying crosslinking, crosslinkers can be more thoroughly mixed with the polymer fluid prior to crosslink initiation, providing more effective crosslinks, more uniform distribution of crosslinks, and better gel properties. Certain embodiments provide better control of crosslinking than certain mechanisms known in the art (e.g., use of delaying ligand for metal crosslinkers).
Furthermore, certain embodiments of the present invention can be used to overcome operational problems at the surface. For example, a wetting surfactant which typically causes problems in pumping due to foaming can be sequestered in the discontinuous phase of the emulsion of a fluid composition of the present invention until its release is triggered by a destabilizing condition downhole.
Delayed release of certain chemicals in various stimulation fluids and workover fluids is also desirable for other reasons. The use of certain methods and compositions of the present invention in hydrocarbon well operations can reduce the damage sustained by tubulars and pumping equipment. For example, mineral acids (e.g., pH modifiers) used in acidizing treatments are corrosive and can cause damage to the tubulars and pumping equipment. If the acid is sequestered and its release delayed until it is in the formation or shortly before it reaches the desired location downhole, damage to pumping equipment and tubulars can be reduced or avoided. Thus, certain embodiments of the present invention are directed to the delayed release of chemicals to minimize damage to pumping equipment and/or well bore structural elements.
Furthermore, using certain methods and compositions of the present invention, improved radial penetration of the hydrocarbon containing formation by certain workover fluids can be achieved. In certain instances, chemical or biological agents, that would normally react in the immediate vicinity of the well bore, are sequestered for a time in an emulsion, allowing them to travel further radially through the formation before reacting.
Another advantage of the present invention is that certain embodiments can be used for slow release of corrosion inhibitors and dissolvers and biocides downhole. Corrosion inhibitors can be slowly released in the well bore to enhance their functionality over time. Biocides can be slowly released to achieve protection over a longer time span, thus requiring lower overall quantities of the biocide.