The EMSU Waterflood Project: A Case History of Infill Drilling, Completions, and Workovers
- R.K. Mitchell (Chevron U.S.A.) | G.S. Salvo (Greenhill Petroleum Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- June 1991
- Document Type
- Journal Paper
- 118 - 124
- 1991. Society of Petroleum Engineers
- 1.14.4 Cement and Bond Evaluation, 1.6 Drilling Operations, 1.10 Drilling Equipment, 4.2 Pipelines, Flowlines and Risers, 1.14.1 Casing Design, 5.4 Enhanced Recovery, 5.6.1 Open hole/cased hole log analysis, 3 Production and Well Operations, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 1.8 Formation Damage, 5.3.2 Multiphase Flow, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.2 Separation and Treating, 5.4.1 Waterflooding, 1.14 Casing and Cementing, 4.3.4 Scale, 4.2.3 Materials and Corrosion, 1.11 Drilling Fluids and Materials, 2 Well Completion, 5.6.4 Drillstem/Well Testing, 5.4.2 Gas Injection Methods, 2.4.3 Sand/Solids Control
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The Eunice Monument South Unit (EMSU) waterflood project is a large-scale (354 wells) example of implementing a waterflood in a highly depleted reservoir. Because of the maturity of this field ( greater than 50 years), several problems were encountered, such as drilling depleted intervals, corroded wellbores, and stimulating depleted and scaled-off formations. This paper discusses procedures to deal with these problems. Other topics discussed include infill drilling techniques, drilling and completion of high-volume water-supply wells, use of air/mist in drilling and deepenings, and water/CO2 injection equipment. By using existing techniques and implementing new ideas, we continually cut costs throughout the life of the project. Operations were improved through continuity, implementation and analysis of new techniques, and good communications. All improvements are documented with field results and show how continual monitoring and adjustments in operations can lead to an operationally efficient, cost-effective project.
The EMSU is currently the largest waterflood in New Mexico, encompassing more than 14,280 acres and 354 wellbores. The unit is located in Lea County, NM, roughly 21 miles southwest of Hobbs (Fig. 1). Unitization of the waterflood began in 1979 and was fully approved by the 21 working-interest owners in Feb. 1985. The unitized interval is defined as a 100-ft subsea datum or the top of the Grayburg pay zone (whichever is higher) and extends through the base of the San Andreas aquifer (Fig. 2). These depths range from about 3,600 (top) to 5,200 (base) ft. The production interval is the Grayburg formation, a dolomite with intermittent sand stringers. Geologic findings showed that five separate lobes of potential pay exist within the Grayburg formation. Below the fifth lobe lies the lower Grayburg and San Andres aquifers, strong waterdrive reservoirs and prolific water producers. The success of the drilling and workover program at EMSU was crucial for two reasons. First, the project budget for this waterflood was more than $60 million, a considerable expenditure during 1985-87. If progress was not substantial and economical, the working-interest owners could have voted to delay or to cancel appropriations. Second, two additional waterfloods, directly north and south of the EMSU, would be based on the success and/or failure of the EMSU project. The two proposed floods were to be similar to the EMSU (same interval and geographic area); therefore, substantial savings could be realized by solving early problems encountered at EMSU.
Scope of Operations
The Eunice Monument field was discovered in 1927. More than 95 % of the 354 existing wells were drilled by 1938. Nearly all these wells were openhole completions, and many contained large cavernous sections as a result of being shot with nitroglycerin. Less than 10% of these wells had usable logs, and many had no well records at all. The unit contained 32 plugged and abandoned wellbores that required evaluation for re-entry. Also, more than 90% of the existing wellbores had to be deepened to expose all the potentially productive Grayburg interval. Upon unitization it was determined that 49 replacement wells had to be drilled. Also, 145 producer and 153 injector workovers were required. Of the 298 proposed workovers, more than 270 required deepening in addition to remedial and stimulation work. Numerous drillstem tests and cores were required to supply geologic and reservoir information about the Grayburg formation. As with any large project, several monumental problems had to be addressed during drilling and workover operations. 1. The existing Grayburg interval contained reservoir pressures less than 200 psi. Controlling lost circulation while minimizing formation damage was a top priority during drilling and workover operations. 2. Large intervals of core had to be cut economically. Also, non-damaged cores were needed to evaluate waterflood performance. 3. Water-supply wells capable of providing 100,000 B/D had to be drilled and completed. 4. In total, 298 wells required cleaning out, deepening, and remedial work. The average age of these wells was 50 years, and all contained H2S and corrosive waters. Almost 25% of these wells contained junk (ranging from cable tools to openhole packers), and well records were poor.
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