Residual oil determination, advances key to program

July 3, 2000
The main keys for determining how to redevelop the Romashkinsky field include proper characterization of the volume and location of the remaining oil reserves and implementation of technologies for recovering these reserves.

ROMASHKINSKY REDEVELOPMENT-Conclusion

The main keys for determining how to redevelop the Romashkinsky field include proper characterization of the volume and location of the remaining oil reserves and implementation of technologies for recovering these reserves.

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This concluding part of a two-part series (Part 1, OGJ, June 26, 2000, p. 42) describes the methods used to determine remaining reserves in the D1D0 horizons, which have produced most of the oil from the field and the additional field development that has lowered water production and decreased the rate of oil production decline.

Reservoir models

The Tatnipineft institute has established a data bank on formation parameters, and histories on:

  • Oil and fluid production from wells and formations.
  • Water injection.
  • Well workovers.
  • Other information on the formations and wells.

The study of the D1D0 horizons included a quasi-3D geologic model LAZURIT.1 This model is complex and allows the construction of maps that show the amount and distribution of remaining reserves in the formations. These maps, prepared for different periods, illustrate the changes in the oil saturation over time.

Also at the beginning of 1997, AO Tatneft acquired RISC-workstations and Landmark software. LAZURIT and the Landmark software complement each other and create a powerful tool for building continuously operating models for AO Tatneft oil fields.

One of the initial projects for the software has been to analyze data on the Romashkinsky field D1D0 horizons. Table 2 illustrates the scale of the models developed.

The modeling and geologic analysis provided ways to:

  • Continue oil production.
  • Decrease associated water production and injection.
  • Establish conditions for long-term well operations.
  • Conduct workover and other jobs.
  • Achieve the most economically profitable oil recovery.

The redevelopment plan also includes drilling new wells for subdividing commingled intervals, optimizing well spacing density, and increasing the number of injection wells. Thus, the number of wells has increased 26% in recent years, and the ratio of producing wells to injection wells has decreased from 3.5:1 to 2.7:1.

The second important adjustment for redevelopment has been implementation of a nonstationary waterflood; in other words, the fluid flow direction is changed in cycles.2 This is an efficient method for decreasing associated water production, and consequently, reducing the water-cut from wells.

AO Tatneft has accumulated a significant amount of experience in nonstationary waterflooding in formations with different geologic conditions and development stages.

In the Romashkinsky field, almost all wells are in a nonstationary waterflooding plan. This cyclic injection amounts to 87.3% of annual injected water.

Estimates indicate that more than 18 million tonnes of additional oil production have been recovered because of this method, while associated water production has decreased to 85 million tonnes.

Some important ways of restricting associated water production are:

  • Shut in high water-cut wells with unprofitable oil flow rates. A shut in of 300-500 wells/year has decreased water production by 25,000 tonnes/day
  • Isolate flooded-out intervals. About 350 intervals have been isolated in 200-250 wells, decreasing water production by 10,000 tonnes/day. Other isolation work in about 250-300 wells/years has decreased water production by another 3,000 tonnes/day.

As a result, annual water production in Romashkinsky field decreased from 180.5 million tonnes in 1985 to 81.3 million tonnes in 1998, while the oil production decline has moderated, as follows:

  • 1992-19.40 million tonnes.
  • 1993-16.63 million tonnes.
  • 1994-15.44 million tonnes.
  • 1995-15.65 million tonnes.
  • 1996-15.14 million tonnes.
  • 1997-14.76 million tonnes.
  • 1998-14.57 million tonnes.

Increasing oil recovery

To increase production from new wells, we now conduct drilling with muds that do not damage reservoir properties. Also in the field, techniques have been implemented that restore and improve fluid flow from the reservoirs after perforating. These measures have improved productivity of new wells by 1.5-2 times.

Plans also include drilling horizontal laterals and sidetracks from existing wells. These completions will likely increase production by 1.5-2.5 times compared with nominal production rates from vertical wells.

Further exploration plans include:

  • Deepening of 275 previously drilled wells to underlying horizons.
  • Drilling of 320 new wells to cover all of the Devonian sands.
  • Delineating the D2-D4 horizons by deepening and sidetracking 761 wells.
  • Analyzing 992 pools in different stratigraphic horizons with a pulsed neutron log (PNG) in 1,252 wells and testing the wells after adding perforations.

Other more unconventional methods also have been used in the field for searching for new oil pools. These include surface geochemical and gamma-spectrometry surveys, biologic and geochemical testing, and neural networks for seismic data processing.

Complex geochemical and gamma-spectrometry surveys are intended to forecast local hydrocarbon accumulations.3 The biological and geochemical testing can indicate the presence of oil pools on structures, especially in tectonic elements without much confirmation from other data. Results obtained to date are good.4

Multilayer neural network computer maps have provided forecasts of oil-and-gas prospects.5 This analysis, tested in the northwestern outskirts of the Romashkinsky field, resulted in drilling Wells 785 and 1027, both penetrating commercial oil pools in Devonian sands.

From this work, the current estimate of the potential oil resources in less extensive oil-bearing horizons of Romashkinsky field is about 668.8 million tonnes of which 149.5 million tonnes are believed to be recoverable.

In addition, there are plans for enhanced oil recovery (EOR), which may increase oil recovery by another 2.5%. EOR methods that are thought to be applicable in the Romashkinsky field include polymer-disperse systems, oxyethylcellulose, polyacrylamide-based (PAM) viscous-elastic systems with chrome acetate, seismic acoustics, thermo-implosion, and microbial.

The redevelopment plans also call for hydraulic fracturing and acid treatment in carbonates to accelerate production rates.7

Well condition

The poor mechanical condition of the wells has been evident since the 1970s. At that time it was noted that the casing in wells drilled from 1950-1970s was cemented only over the main producing interval. This casing began to fail because of corrosion.

Wells completed after the 1970s had longer cement columns behind the casing and were cathodically protected.

Some wells with a large number of casing failures were abandoned and new wells replaced them. Currently, more than 700 replacement wells have been drilled.

All new wells have a complete cement column behind the casing, and service life has increased by 2-3 times.

A new water-treatment system has improved the quality and volume of the injected water. The previous high-capacity pumps have been replaced by lower capacity pumps that allow different injection pressures for areas with different reservoir characteristics.

Some other producing problems that have been mitigated include:

  • Water line and wellhead freezing in winter during times of low injection volumes.
  • Asphaltene-resin-paraffin deposits in marginal wells.
  • Pumping unit failures by increased overhaul of pumping equipment.

One technology introduced in many wells by the oil production enterprise Leninogorskneft restricts associated water production from marginal wells. This method maintains formation pressure in wells at a static level, and oil flowing from the formation is periodically recovered by pumping units with pumps set at higher depths.

Environment

AO Tatneft, in the last 5-10 years, has also worked on solving various ecological problems to conserve resources, prevent damage to oil production facilities due to corrosion, and protect the water basins, land, and atmosphere.

It has also investigated the possible influence of oil field development on seismic activity in southeast Tatarstan.

To protect the environment, it appoints special supervisors and allocates material, technical, and financial resources for implementing these measures.

Because of this program:

  • The accident rate has decreased.
  • Harmful effects of producing and pumping large volumes of formation and wastewater has decreased due to its restricted production.
  • Light hydrocarbon fractions from the associated gas are being recovered.
  • In inhabited areas the quality of drinking water has improved by sealing off springs from the produced and wastewater discharges.

To solve corrosion problems, AO Tatneft created its own pipe-coating facilities. Currently it is a major manufacturer of corrosion-resistant pipe. Its facilities have the capacity to apply inside and outside coating to about 4,000 km of pipe/year.

Remaining research

Investigations are being conducted on anaerobic microbes in the waterfloods that on one hand contribute to hydrogen sulfide formation in waters and consequently increases corrosion, but on the other hand can reduce oil viscosity by changing higher molecular oil fractions into lighter fractions.

Also, quality of injected water is still a problem. In highly productive reservoirs, poor water quality can improve sweep efficiency by plugging large porous channels; but for low-permeability zones the water in unsuitable for injection.

Different norms of water quality for different class rocks have been developed.

AO Tatneft has not come to a consensus regarding the influence of clay swelling on decreased flow rates while injecting freshwater. Some investigators believe that the illite in most samples is present in such small quantities that it cannot significantly change reservoir permeability. These minerals are also distributed as separate accumulations and are not uniformly distributed across the reservoir.

Other investigators argue that clay minerals significantly affect the reservoir permeability depending on chemical composition of water pumped through reservoir.

Having insufficient reservoir samples for direct laboratory measurements hinders studying this problem. The clay indices according to radioactive well logs are often distorted because of the presence of radioactive elements in the formations.

Injection of large volumes of cold water into formation will undoubtedly lead to negative consequences. Conducting this process, however, by taking into account the heat from the formations at depth can decrease this problem.

Therefore, despite injecting more than 6 billion cu m of cold water into productive formations, the formation temperature in the Romashkinsky field has not decreased. Wax-precipitation temperatures have not been reached in any area, although formation temperatures in the region of injection wells decreases in winter to sometimes less than 10-15 degrees C.

For the last 5 years, TO Tatneft has investigated the influence of the formation pressure decrease on reservoir properties and oil recovery factors. The results indicate that negative influence, although possible, is insignificant within the accuracy of recoverable oil reserve estimates. But changes in near-well stress created by drilling can significantly decrease well productivity.

References

  1. Muslimov R.Kh., Khisamov R.S., and Suleimanov E.I., et al., "Creation of continuously operating models of Romashkinsky and Novo-Elkhovsky fields in the Republic of Tatarstan based on automatic workstation "LAZURIT" and Landmark software package," (In Russian), Nedra, Neftyanoye Khozyastvo, No 7, 1998, pp.63-67.
  2. Akhmetov Z.M., Galeev R.G., and Diyashev, R.N., "Improvement of Technology on Enhanced Oil Recovery Using Methods of Non-Stationary Water Flooding by the Example of Romashkinsky Field," 9th European Symposium on Improved Oil Recovery, Hague, Oct. 20-22, 1997.
  3. Galeev R.G., and Muslimov, R.Kh., "The state of oil industry in Tatarstan and methods of highly efficient development of fields at late stage of oil resources production," Neftyanoye Khozyastvo, (In Russian), No 12, 1995, pp. 26-33.
  4. Diyashev R.N., Gottikh R.P., and Larochkina I.A., "Integrated Methods for Geochemical Prospecting of Oil and Gas Fields," Muslimov R.Kh., 2nd Middle East Geosciences Conference & Exhibition, Bahrain, Apr. 15-17, 1996. GeoArabia, Vol. 1, No.1, March 1996.
  5. Patent RF No 2094828, Oct. 27, 1997.
  6. "Concept of EOR Methods Development," (In Russian), Bugulma, May 27-28, 1996.