STRIPPER WELL PRODUCTION INCREASED WITH MEOR TREATMENT

Sidney J. Nelson, Phillip D. Launt
RAM Biochemicals Inc.
Raleigh, N.C.

Stimulation of single wells by microbial injection has proved to be an effective technique for improving marginal oil production.

Microbial stimulation is particularly suited for stripper wells because of the relatively low cost for process development and testing, and even lower cost for subsequent well treatments.

The U.S. is estimated to have had approximately 500 billion bbl of oil in place before production was first begun. Primary recovery driven by existing reservoir gas pressure and natural water movement will retrieve 12-15% (approximately 75 billion bbl) and secondary recovery by waterflooding another 100 billion bbl. That leaves 325 billion bbl unproduced.

Enhanced oil recovery (EOR) methods are estimated to allow recovery of an additional 10% of the original oil in place (50 billion bbl) at costs ranging from $10 to $45/bbl.1 This leaves approximately 275 billion bbl unrecoverable by current and advanced EOR processes.

For this reason, an emerging technology that uses micro-organisms in a variety of novel approaches has received renewed interest worldwide. This recovery method is known as microbial enhanced oil recovery or simply MEOR, and it offers great promise from the standpoint of both cost and recoverability of the remaining oil.

MICRO-ORGANISMS DOWNHOLE

Micro-organisms are always present in oil wells. They can help or hinder oil production in many ways.

These microbial communities are a combination of indigenous organisms and air and water-borne organisms (contamination), the result of initial drilling, and secondary and tertiary recovery operations.

Oil recovery can be hindered by micro-organisms that produce slime exudates called biofilm that plug reservoir pores.

Steel casing, pipe, and pumps are attacked by corrosive chemicals produced by sulfate-reducing bacteria that cause sweet crude to turn sour.

When hydrogen sulfide gas is formed by reduction of sulfate, it reacts with iron in the reservoir to form ferrous sulfide, a heavy black precipitate often responsible for plugged and fouled production lines.

Micro-organisms can also cause degradation of chemicals used in EOR processes. But fortunately, not all microbial activity hinders oil production.

Micro-organisms produce a wide range of metabolic byproducts that are helpful to oil recovery. Additionally, some anaerobic micro-organisms produce hydrocarbon gases, and some aerobic organisms produce carbon dioxide gas.

Both types of gas can saturate well head oil and help repressurize the formation. Still other organisms can upgrade crude oils by removing sulfur or nitrogen-containing compounds.2

MEOR HISTORY

MEOR is still considered an emerging technology, but it is certainly not new. Both micro-organisms and microbially derived products have been used for over 40 years to recover reservoir oil or provide well bore cleanout.

The suggestion to use micro-organisms to increase oil production was first made by J. W. Beckmann in 1926.3 But systematic laboratory investigations were not begun until the 1940s, and field trials were not started until the early 1950s.

A pioneer in the laboratory study of MEOR was C. E. ZoBell who headed the American Petroleum Institute's Research Project 43A. Reports of these laboratory investigations first appeared in August 1947.4 5

MEOR field trials were begun by both the U.S. and the U.S.S.R. in the early 1950s, followed by Czechoslovakia in the late 1950s, Poland early in the 1960s, Hungary in the mid-1960s, and Romania in the early 1970s.

The U.K., Canada, Australia, Germany, and China have also conducted MEOR research, field trials, or both. All of these countries are continuing the development of their MEOR technology.

Presently, the focus of U.S. government and university-sponsored MEOR work is on understanding the basic mechanisms of MEOR. To this end, the U.S. hosted the First International Conference on Microbial Enhancement of Oil Recovery, held near Afton, Okla., in 1982. Several more have been held to date.

THE PRIVATE SECTOR

Major oil companies and private high-tech laboratories are similarly engaged in MEOR research and development, but much greater visibility is afforded the field applications of their MEOR products or processes.

While proprietary interests have kept their research and development work out of the public domain, except when patent applications are made, field trial results are disseminated openly.

The private sector's motivation is profit, not curiosity, and the emphasis is on field trial results rather than on understanding the MEOR mechanism. This is not surprising as both cyclic MEOR injection projects and enhanced waterflooding have yielded positive results.

In addition to RAM Biochemicals Inc., other companies with on-going MEOR field projects are Injectech Inc., Alpha Environmental, Petroleum Bioresources, Microbial System Corp., Petrolite Corp., Microbios Ltd., Microbes Research & Development, and Geo-Microbial Tech Inc.3

This listing is by no means complete.

MEOR APPROACHES

A variety of MEOR approaches have been used including:

Stimulation of the existing (indigenous) microbial populations
Injection of nutrients, biocatalysts, and selected micro-organisms with a proven ability to perform in situ
The aboveground production of microbial products, such as biosurfactants, biopolymers, fermentation alcohols, and ketones which are injected like traditional EOR chemicals.6

This article deals only with the second approach.

CYCLIC MICROBIAL RECOVERY

Using a well to inject nutrients, biocatalysts, and selected micro-organisms, and then later to produce, is known as cyclic microbial recovery or, in field terminology, the bio "huff and puff" method (Fig. 1).

A shut-in period (incubation period) ranging from days to weeks is required for microbial growth. The length of the shut-in period is determined primarily by downhole temperatures.

A production phase follows the shut-in period, and the cycle is repeated when production falls off significantly. An example of the results offered by this method is shown in the gross production of the Burnett J lease (Fig. 2).

Three wells are on the Burnett J lease. The lease has a history of moderate to heavy paraffin buildup that requires pulling and steam cleaning of sucker rods.

The production is from a 60-ft thick granite wash at 3,255 ft. Downhole temperature is 80-85 F., porosity is 14.6%, and permeability is 28 md. The produced oil is 39.5 API gravity.

Before the treatment, the lease was producing at 0.3 bo/d. After treatment, the production increased to 1.6 bo/d. Gas and water production remained the same, 13 Mcfd and 8 bw/d.

The treatment consisted of injecting 20 gal of kerosine into the well bore as a preflush followed by about 15 gal of RAM Biochemicals' WelPrep 5 and a flush with 2 bbl of salt water (lease water). The well was shut in for 4896 hr and then returned to production.

WELL BORE CLEANOUT

When cyclic injections are not repeated at regular intervals, the result is often indicative of a microbial well bore cleanout. The Schlobohm Tucker, P.P. Karlin, and HW-W leases are examples (Table 1).

MICROBIAL FLOODING

Microbial flooding utilizes the effect of microbial solutions on a reservoir. An injection and recovery well pattern is established, and a solution of micro-organisms, nutrients, and biocatalysts is injected.

As this solution is pushed through the reservoir by drive water, it forms gases and microbial products that help release and mobilize the oil which is then pumped out through the production wells.

MEOR MECHANISMS

It has long been established that micro-organisms produce a wide range of metabolic by-products helpful to oil recovery, but only recently have biotech firms developed products containing selected micro-organisms for use in EOR.

No single type of microorganism will do everything. But in this regard, it is important to remember that most MEOR methods use a mixed microbial population. That is, a variety of different organisms are used rather than one single type. The sum total of their activity is therefore likely to include:

The production of gases (CO2, H2, N2, CH4) which can increase reservoir pressure and reduce oil viscosity
The production of oil-releasing metabolites (e.g., low molecular weight alcohols, organic acids, and biosurfactants) which release oil from reservoir rock
The production of polymers to facilitate mobility control or for selective or nonselective plugging
The production of solvents which dissolve oil.

SWEET APPROACH

Most of the work in MEOR to date has been directed towards micro-organisms that are grown on carbohydrate nutrients. Once introduced downhole, large amounts of nutrients, like molasses or 2% whey solution, must be either continuously or regularly injected to maintain their growth.

While increases in oil production quite often result, the CO2 produced by fermentation robs BTU value from any coproduced gas. It is usually removed cryogenically, further adding to production expenses.

Additionally, the cost of this approach is dependent in large measure upon continued low sugar prices.

RAM'S APPROACH

From the beginning, RAM Biochemicals focused its attention on anaerobic bacteria that could utilize hydrocarbons in the reservoir, in situ, and produce biogas (methane or ethane) rather than CO2.

This eliminates the need for injecting large volumes of carbohydrate nutrients and avoids the loss of BTU value in any coproduced gas. Other factors influencing this decision were:

Reservoirs are generally lacking in oxygen or anaerobic.
Anaerobic bacteria can tolerate extremes of temperature, pH, salinity, and solvent contamination.
Anaerobes produce a large number of oil-releasing metabolite products.

There are drawbacks associated with this approach to be sure. One is the volume of gas produced, which is generally lower than with carbohydrate fermentation approaches.

Of greater concern to RAM was the possibility that sulfate-reducing bacteria would be introduced downhole. By the corrosive chemicals bacteria produce, steel casing, pipe, and pumps could be attacked.

Yet another potential problem was the inadvertent production of hydrogen sulfide (sour) gas.

Fortunately, none of these problems has occurred on any of RAM's field projects to date.

RAM Biochemicals developed Wel-Prep 5, an oil recovery fluid which can be used for:

Microbial well bore cleanouts
An MEOR treatment fluid for cyclic injection
An MEOR fluid for enhanced water flooding.

The product is a liquid preparation containing selected micro-organisms, biosurfactants, nutrients, and biocatalysts. It was developed for tertiary recovery of residual reservoir oil. Its anaerobic, hydrocarbon-utilizing organisms are meant to become established downhole and then be periodically stimulated with retreatments, thus beginning and maintaining an in situ process where biochemicals helpful to oil recovery are continually manufactured.

CANDIDATE WELLS

As far as we know, no method has yet been devised for predetermining which wells will respond to microbial treatment. There are simply too many variables and uncontrollable factors involved to make an absolute prediction.

Therefore, the screening process or approach favored by most people working in MEOR is to first locate a number of wells having conditions favorable for microbial growth, then treat several of them and monitor their progress closely.

A maintenance level retreatment schedule based upon the pattern of response is developed for those wells showing a positive response. Wells that do not respond after two treatments are dropped from the program and new candidates sought.

Typical candidate wells have the following characteristics:

Production, stripper well up to 20 bo/d
Temperature, 20-60 C. (85-140 F.)
A ph of 4-9
Permeability, 20 md
API gravity, 20
Porosity, 3%
Depth (typical), 1,000 m (3,200 ft).

ACTUAL TREATMENTS

In November 1989, a Kansas producer, Starr F. Schlobohm Oil Operations of Russell, Kan., began cyclic injections of Well-Prep 5 in six of its single-well leases. Three of the six wells have responded positively, none negatively (Table 1).

Another Kansas producer, American Warrior Inc., Garden City, Kan., began its program in December 1990. Production increased by 1.1 bo/d, from 8.7 to 9.8 bo/d.

In Texas, L. Jack Gross & Alstar Production began cyclic treatments in January 1988. As of Dec. 31, 1990, 26 wells were being treated on a regular basis. Production increased from 670 bo/month to 812 bo/month.

In Illinois, Greater Midwest Oil Co. Inc., Mt. Vernon, Ill., began its program in November 1990 on the Thompsonville Pool in Hamilton County, Ill. Production in Well Osbourn No. 2 increased from 2 to 4 bo/d and in the No. 4 from 3 to 6 bo/d.

ECONOMICS

Traditional EOR and most MEOR treatment processes are costly in relation to the additional barrels of oil they recover. Costs range from $10 to $46/bbl of oil produced. Injection costs are $335/bbl.7

This cost is not for the incremental oil or the amount of oil produced which falls above a calculated decline curve, but actual production increases over and above what the well or field has averaged for the previous 612 month period.

These cost figures generally hold true for all EOR and MEOR processes except those MEOR processes which use in situ methods and do not require large amounts of regularly injected nutrients. Wel-Prep 5 stripper well treatments by cyclic injection range in cost from $0.90 to $10.00/bbl of additional oil production.8

TAX CONSIDERATIONS

Although MEOR is recognized as a tertiary recovery method in every authoritative text on enhanced oil recovery, it remains a black hole for the various state and jurisdictional agencies charged with determining whether or not a project qualifies as a tertiary recovery project. Qualified tertiary oil recovery projects do receive special tax considerations at the county, state, and federal levels.

The method of calculating tertiary tax considerations and their actual dollar amounts varies from state to state, but county ad valorem taxes, state severance taxes (bo/d allowable per well), and state excise taxes are all affected.

So too are federal excise taxes and the federal itemized deductions for tertiary injectant expenses.

These tax savings should not be overlooked as a means of cutting expenses and increasing profitability, as they offer operators substantial advantages.

In Kansas, for example, $1,500-1,700 yearly savings per well can be realized from decreased state severance tax alone.

REFERENCES

Leonard, J., "EOR set to make significant contribution," OGJ, Apr. 2 1984, pp. 83-105.
RAM Biochemicals Inc., Wel-Prep 5-Product Information & Guide, June 28, 1990.
Tech Fu Yen, Microbial Enhanced Oil Recovery: Principle & Practice, CRC Press Inc., 1990.
ZoBell, C.E., "Bacterial release of oil from oil-bearing materials (Part 1), World Oil, Aug. 25, 1947, pp. 36-47.
ZoBell, C.E., "Bacterial release of oil from sedimentary materials," OGJ, Vol. 43, No. 13, pp. 62-65.
Finnerty, W.R., and Singer, M.E. "Microbial Enhancement of Oil Recovery," Biotechnology, March 1983, pp. 47-54.
Westlake, D.W., Petroleum Microbiology, Elsevier, N.Y., 1967, p. 545.
RAM Biochemicals Inc., Field Projects, 1987-1990-Limestone, Chalk, Chalk/Saturated Brine Strata, Granite Wash/Acid Brine Strata, and Sandstone Strata, in Texas, Kansas, Oklahoma, and Missouri.