Method for recovering gas from shale reservoirs and purifying resulting produced water to allow the produced water to be used as drilling or frac water, or discharged to the environment

A system and method for removing gas from a gas producing subterranean formation and removing contaminants from the produced water is provided. The method includes providing a mixture of gas and water gathered from a gas producing well. Thereafter the method entails separating the gas from the mixture to produce a gas product and the produced water which includes organics, suspended solids and dissolved solids including silica. Thereafter, suspended solids are removed from the produced water. After removing some of the suspended solids, the produced water is directed to a micro porous polymer extraction (MPPE) unit. In the MPPE unit aromatic organic compounds are removed from the produced water. Thereafter the produced water is treated in a membrane bioreactor to remove additional organics. Thereafter the produced water is directed to at least one RO unit that produces a high dissolved solids containing reject stream and a low dissolved solids permeate stream. The method or process described reduces the concentration of certain contaminants in the produced water such that the output or permeate stream from at least one RO unit can be used for frac fluid, drilling mud, or discharged to the environment.

FIELD OF THE INVENTION

The present invention relates to recovering gas or oil from subsurface reservoirs and purifying the resulting produced water.

BACKGROUND OF THE INVENTION

Natural gas is sometimes buried below the Earth's surface in shale reservoirs. To recover this natural gas, the shale reservoirs are hydraulically fractured with large volumes of frac fluid or gel to create a path for the natural gas to flow to the surface. Often, shale reservoirs are associated with ground water. When the natural gas flows to the surface for recovery, the natural water and the water in the frac fluid (sometimes referred to as frac flow back water) flows to the surface as well. The recovered natural water and the water used to frac the shale reservoir, along with oil in some cases, are separated from the natural gas. This water separated from the natural gas is referred to as produced water.

Produced water is often contaminated with concentrations of constituents requiring the produced water to be treated before it can be reused or discharged to the environment. Produced water typically includes natural contaminants that come from the subsurface environment. These contaminants include hydrocarbons from oil and inorganic salts. Produced water may also include manmade contaminants such as drilling mud, frac flow back water that includes spent fracturing fluids, including polymers and inorganic cross-leaking agents, polymer breaking agents, friction reduction chemicals, and artificial lubricants. These manmade contaminants are injected into wells as part of the drilling and production processes and recovered as contaminants in the produced water. In addition, the produced water may include contaminants that result from drilling mud. These contaminants may include acids and caustics such as soda ash, calcium carbonate, sodium hydroxide, and magnesium hydroxide. Furthermore, the contaminants in the produced water may include defoamers, emulsifiers, filtrate reducers, shale control inhibitors, deicers including methanol and other contaminants.

One of the challenges in treating produced water resulting from shale gas production and other natural gas production is to develop a practical and cost effective process that purifies the produced water such that it is suitable for use as frac water, in drilling mud, or which could be discharged to the environment.

SUMMARY OF THE INVENTION

The present invention relates to a method for removing gas from a subterranean reservoir and treating the resulting produced water. Generally the produced water includes organics, suspended solids, dissolved solids and other contaminants. In the present process, in one embodiment, the suspended solids are separated and removed from the produced water. Organics are removed in both a macro porous polymer extraction unit and a membrane bioreactor (MBR). Effluent from the MBR is directed through at least one reverse osmosis (RO) unit and optionally through a second reverse osmosis unit. The first reverse osmosis unit removes contaminants such as dissolved solids from the produced water. Permeate from the first reverse osmosis unit is sufficiently pure that it is suitable for use as frac water or in drilling mud. In some cases the produced water will include a significant boron concentration. The optional second reverse osmosis unit is effective to remove boron and hence the permeate produced by the second reverse osmosis unit is sufficiently pure to be discharged to the environment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

There are many gas fields in North America and other places in the world that contain substantial quantities of natural gas. This gas is typically trapped in shell formations that require stimulating the well using a process known as fracturing or tracing. A fracing process uses large amounts of water and substantial amounts of particulate fracing material, commonly referred to as frac sands, to enable the extraction of gas from the shell formations. After the well site has been stimulated, the water pumped into the well during the tracing process is removed. The water removed from the well is often referred to as flow back fluid or frac water. A typical fracing process might use a few million gallons of water to fracture the formation of a single well. Since water is an important natural resource that should be conserved whenever possible, should be taken to conserve water by treating the frac water and recycling it.

The present invention is directed to a system and process for recovering shell gas and treating the resulting produced water such that the water can be used as a part of the frac fluid, or used as a drilling mud, or even discharged to the environment.

Before discussing the process in detail, it is beneficial to basically review the components of the system used in removing gas and treating the resulting produced water. Shown inFIG. 1is a schematic illustration of the gas recovery and produced water purification system of the present invention. The process and system shown therein is indicated generally by the numeral10.

First the system and process10includes a gas-water separation unit12. Gas-water separation unit12is a conventional subsystem that receives a gas-water mixture from the producing well and separates the mixture into gas and produced water. The resulting gas is collected by conventional means. This leaves the produced water to be treated so as to remove certain contaminants to a concentration level that permits the produced water to be used as frac water, drilling mud, or even discharged to the environment.

As seen inFIG. 1, the system and process includes numerous chemical injection sites for injecting selected chemicals into the system for treating the produced water. Downstream from the gas-water separation unit12is a coagulant injection site14and an acid injection site16. Various coagulants and acids can be used, but in one exemplary embodiment the coagulant selected is ferric chloride and the acid utilized is sulfuric acid.

Downstream from the injection sites14and16is a solids separator18. The function of the solids separator18is to remove suspended solids including any precipitants that result from upstream chemical treatment. Various types of solids separators can be utilized. In the embodiment shown inFIG. 1, the solids separator18is a dissolved air flotation (DAF) unit. Other types of solid separators such as clarifiers, induced gas flotation (IGF) units, flocculation systems and ballasted flocculation systems can be employed to remove suspended solids and precipitants.

Downstream from the solid separator18is a pair of macro porous polymer extraction (MPPE) units. One MPPE unit is referred to by the numeral20and the other MPPE unit22is referred to by the numeral22. In the exemplary process shown herein, one of the MPPE units is active in treating the produced water while the other MPPE unit assumes an inactive mode. In the inactive mode the MPPE unit assumes a regeneration state.

After the produced water exits the MPPE unit, the produced water is directed to a membrane bioreactor (MBR)24. MBR24includes a treatment tank24A and a filtration tank24B. In the filtration tank is a plurality of membranes24C that in the exemplary embodiment discussed herein are ultrafiltration membranes. Other types of membranes can be used. MBR24includes a sludge outlet24D and an effluent line24E. Effluent line24E functions to direct a permeate stream of produced water from the MBR24.

Downstream from the MBR24is another injection site26which in this case is utilized to inject a chemical that inhibits the precipitation of silica.

As seen inFIG. 1the system can include various pumps28and34that are utilized at various positions in the process to pump the produced water downstream. In particular pump28pumps the produced water to a first RO unit indicated generally by the name of30. RO unit30includes a reject line30A and a permeate line30B. Treatment of the produced water in the first RO unit or the first RO pass produces a reject stream having a relatively high dissolved solids content that is directed through reject line30A to an upstream point in the process. Permeate produced in the first RO unit30has a relatively low dissolved solids content and is directed out line30B. As will be discussed subsequently herein, in some cases the permeate produced by the first RO unit30is of a quality that enables it to be used as frac fluid, drilling mud or even discharged to the environment. This is illustrated inFIG. 1. There are, however situations where the permeate produced by the first RO unit30has not been sufficiently treated in order to permit it to be discharged to the environment. As will be discussed subsequently herein, the presence of significant amounts of boron in the permeate produced by the first RO unit30will in many cases prevent the produced water at that point from being discharged to the environment. Hence in this case the produced water exiting the first RO unit30must be treated by a second RO unit36. Here there is an additional injection site32disposed between the first RO unit30and the second RO unit36. As will be discussed subsequently herein, injection site32enables a caustic such as sodium hydroxide to be injected and mixed with the produced water so as to raise the pH sufficiently to maintain boron in solution. This enables the boron to be rejected by the second RO unit36which includes a reject line36A and a permeate line36B. In some embodiments, the permeate produced by the second RO unit36is further treated. As shown inFIG. 1there is an injection site38for delivering an acid such as sulfuric acid to the purified produced water and another injection site40for adding another chemical such as calcium sulfate. The significance of these two chemicals will be discussed subsequently herein.

Typically, the resulting produced water separated from shale gas will contain about 15,000 ppm of dissolved solids, and about 3,000 ppm of organic matter measured as total organic carbon. These concentrations can of course vary. Typically such a well produces 8,000 to 10,000 barrels of produced water which could be used as frac fluid or drilling mud if the alkalinity and chlorides could be substantially reduced. As a general rule, water can be discharged to the environment if the total dissolved solids are reduced to less than 1,000 ppm and substantially all of the organics removed.

In some cases, the produced water is recovered and transferred to holding tanks that might be located remotely from the producing well which yielded the produced water. In many cases, the free oil will be skimmed off in these holding tanks. In cold environments, the produced water is heated to 6° C. typically by a gas-fired heating system. Typically in these cases, heat exchangers are employed downstream to recover heat from the produced water and the recovered heat is used upstream to heat the produced water.

After the produced water is separated from the gas, the process aims to remove free oil and suspended solids. In the exemplary embodiment shown inFIG. 1, an acid such as sulfuric acid and a coagulate such as ferric chloride is added to the produced water at injection sites14and16. Typically the pH of the produced water separated from the gas is 7.0 or higher. Adding the sulfuric acid reduces the pH and in the exemplary process discussed herein, it is contemplated that the pH of the produced water will be reduced to approximately 6.7 or below. The sulfuric acid will also neutralize the alkalinity of the produced water. Specifically, the bicarbonate alkalinity will be converted to carbon dioxide and the carbon dioxide will be stripped off to the atmosphere downstream when the produced water is subjected to aeration. Further, by adding the acid, the buffering capacity of the water is reduced, and in cases where a second RO pass is required, it follows that a less amount of caustic is required to sufficiently raise the pH to approximately 10 or above to improve boron rejection in the second RO unit36.

In addition to adding the acid, a coagulate such as ferric chloride is added. The coagulate tends to destabilize suspended solids, free oil, and weak organic acids.

After the acid and coagulate are added by injection sites14and16, the produced water is directed to a solids separator. In the example shown herein, the solids separator is a DAF unit18. It is appreciated that other types of solids separator could be used. In some cases the produced water will include barium and strontium. These chemicals are foulants and will tend to foul the membranes utilized in the downstream RO units30,36. Because sulfuric acid was added to the produced water, barium and strontium precipitate as sulfates and are removed in the DAF unit. In some cases, aluminum is present in the produced water. Aluminum will precipitate as a hydroxide. Thus, in the DAF unit, free oil, weak organic acids, suspended solids, barium, strontium, and aluminum will be removed at least to some extent.

After the free oil, some organic acids, suspended solids and other precipitants are removed, the effluent from the DAF unit is directed to a macro porous polymer extraction (MPPE) unit. As discussed above, in the process shown inFIG. 1, there are two MPPE units20and22. One is operated in an active state while the other generally assumes a regeneration or ready state. Details of the MPPE unit are not dealt with herein because such is not per se material to the present invention, and further, MPPE units are known and are commercially available. The MPPE unit will remove aromatic and poly aromatic organics from the produced water. As used herein, the term “aromatic organics” includes all forms of aromatic organics including poly aromatic organics. The MPPE units20,22are liquid extraction devices. Each MPPE unit includes a column having an array of beads that comprise a polymer that encases or holds an extraction fluid. As the produced water passes over the beads, the extraction fluid extracts organics, particularly aromatic organics, from the produced water. The MPPE unit is effective to reduce the concentration of total organic carbon (TOC). Generally the MPPE unit will not remove all organics, but is effective to remove some. Remaining organics in the produced water can be biologically digested by the MBR24. However, some organic compounds are biotoxic or refractory to biological treatment. One of the purposes of the MPPE unit is to remove these toxic organic compounds from the produced water before they reach the MBR24where they can have an adverse impact on the biomass that is responsible for treating the produced water in the MBR.

As noted above, in a typical example, one of the MPPE units20or22is removed from service in order that the polymer beads can be rejuvenated. This occurs when the extraction fluid in one of the MPPE units becomes saturated with organics. One process for rejuvenating the polymer beads is to use steam produced by a small electric steam generator to strip the media. The steam will remove the organics from the extraction fluid to form a steam-organics mixture. Thereafter, the steam-organics mixture can be condensed and the organics separated from the condensed water which can be recycled to a point upstream in the process.

After the water is treated in one of the MPPE units, the produced water is directed to the MBR. Organic compounds remaining in the produced water are oxidized by aerobic bacteria and converted to carbon dioxide. Weak organic acids that can foul the RO membranes will also be reduced. Oxygen in the MBR will also oxidize iron in the produced water and will cause the iron to precipitate as hydroxide. Bacteria and precipitated iron will be filtered out in the filtration tank24B and the resulting sludge will be directed from the MBR via the sludge outlet line24D. One of the principal RO foulants is high molecular weight organic acids. These are weak acids that do not ionize well and have a limited solubility in water. Typically, if these high molecular weight organic acids are not removed from the produced water prior to reaching the RO units, the RO membranes will reject the organic acids and they will become so concentrated that the organic acids precipitate onto the membrane, and as a result, foul the membranes. In the MBR unit, bacteria will consume these acids and prevent downstream fouling from occurring due to these high molecular weight organic acids. Also, in many cases the produced water has a substantial amount of methanol. RO membranes do not typically remove methanol. In the MBR24, bacteria consume the methanol and generally prevent the methanol from reaching the RO units.

The effluent, or water treated by the MBR, is then directed to an RO system. In the exemplary embodiment shown inFIG. 1, the RO system comprises a two pass system, including RO unit30and RO unit36. Because of the high total dissolved solids in the produced water, the first pass in some exemplary embodiments can benefit from a two stage arrangement. In the first stage of the first RO unit30, the membranes may be what is sometimes referred to as brackish water membranes. In a typical process, the first stage of the first RO unit30will convert about half of the produced water to product water. In one example, the first stage can be operated at approximately 450 psig. The reject from the first stage will then be directed to a booster pump which will increase the feed pressure to the second stage to approximately 800 psig or above. The second stage can be equipped with what is sometimes referred to as seawater membranes. In the end, the first pass RO unit30will operate at approximately 75% recovery. The product water from the first pass or from the first RO unit30is generally of a sufficient quality to be used for frac water or drilling mud. In some cases, the product water of the first RO unit30does not include any significant amount of boron, and in those cases the product water from the first RO unit can be discharged to the environment. In some cases, however, the product water from the first RO unit30will include a boron concentration of about 4 ppm and higher. In that case it will be necessary in most situations to remove the boron, or substantially all of the boron before the product water can be discharged to the environment.

The pH of the produced water plays an important part on the passage of many ions through an RO membrane. Some ions such as ammonia are removed better at low pH. Others are more efficiently removed at high pH. Boron, which is present in many produced waters, is not removed efficiently at low pH. The concern with raising the pH prior to the first RO unit30is that the produced water at that stage includes a significant concentration of calcium carbonate (CaCO3) which would scale the membranes of the first RO unit30. Since substantially all (generally equal to or greater than 99%) of the calcium carbonate is removed by the first RO unit, the pH of the permeate to the second RO unit can be raised without a significant risk of fouling due to the calcium carbonate. To remove the boron, the pH of the feed to the second RO unit36is increased to approximately 10 or higher. Because the buffering of the produced water has been nearly eliminated at this point in the process, it will usually take a relatively small amount of caustic to raise the pH. Since the total dissolved solids of the feedwater to the second RO unit36is low, the second pass or the second RO unit will operate in a typical exemplary process at a little over 200 psig with an expected 90% recovery. To conserve water and minimize waste, the reject water from the second RO unit36is returned to the first RO unit. It is contemplated that in an exemplary process, that the overall recovery of both RO systems taken together will be approximately 73% or higher.

It is appreciated that one of the main concerns in designing RO systems is to provide a process design that eliminates or minimizes RO membrane fouling. The present system and process is designed to eliminate, or at least minimize, membrane fouling of the RO units30and36. Generally, the organics are removed by the DAF, MPPE, and MBR units. Scaling potential due to calcium carbonate is substantially reduced by the addition of the sulfuric acid and can be further controlled by an anti-scalant feed. Iron and aluminum have also been removed upstream from the RO units30and36. This leaves as the major concern, silica. The solubility of silica is about 120 ppm in neutral water. The produced water generally has as much as 70 ppm of silica. At 75% recovery the silica can concentrate up to about 280 ppm. That is more than double saturation. The present invention contemplates adding a silica precipitation inhibitor via injection site26to the produced water upstream of the first RO unit30. Various silica inhibitors can be utilized. One example is a commercial silica precipitation inhibitor known by “Hydrex 4109”. Such silica precipitation inhibitors will generally prevent silica, calcium carbonate, barium sulfate, and a number of other precipitants from forming. However, such chemicals are not generally effective with respect to iron or aluminum. As noted above, removal of aluminum and iron is addressed by the DAF unit and the MBR unit. Generally it has been found that if the produced water is free of iron and aluminum that a silica precipitation inhibitor can maintain silica in solution up to over 300 ppm.

It is appreciated that the makeup or chemical composition of the produced water can vary from location to location. Further, the reduction in concentration of certain constituents can vary from process to process depending on the makeup of the produced water, reagents added, quantity of reagents added, and the effectiveness of various processes utilized. Table 1 appearing below is one example of a produced water stream treated by the method described above. Table 1 shows the makeup or chemical composition of the produced water which includes the contaminants or impurities of interest. Table 1 also indicates the concentrations of these various contaminants or impurities throughout the process.

It is contemplated that the process shown inFIG. 1and described above will produce a product water from the second pass RO unit36that contains 3 ppm or less of total dissolved solids and 0.5 ppm or less of boron. Typically the pH of the product water will be adjusted with a relatively small amount of sulfuric acid and a small amount of calcium sulfate. The calcium sulfate is typically added to increase the sodium absorption ratio to greater than about 6.

As discussed above, the permeate produced by the first RO unit30is typically of a quality that enables it to be used as a constituent of frac fluid or drilling mud. That is the permeate produced by the first RO unit30is of a sufficient quality that it can be used as frac fluid or drilling mud without having to undergo treatment in the second RO unit36.

Table B appearing below shows exemplary requirements for frac water and drilling mud.

It follows that the produced water recovered from a producing well can be treated with the present process to produce product water that can be combined with conventional constituents to make up frac fluid or drilling mud and the resulting frac fluid or drilling mud can then be used in connection with other wells in the vicinity. That is, the resulting frac fluid can be utilized to fracture shell formations and the drilling mud can be utilized in drilling operations. It also follows that the product water produced by this process can be containerized and transported to other sites for use in frac fluid and drilling mud. As pointed out above, if the produced water includes a significant amount of boron, in most cases it will be necessary to remove the boron through the second pass RO system. In any event, if the boron concentration is 0.5 ppm or less then the treated produced water can be discharged to the environment.

The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.