Stylolites Impact the Miscible Nitrogen Flood in a Mature Carbonate Oil Field

Abstract Stylolites can be an important geologic feature affecting reservoir quality and, consequently, reservoir management, in many carbonate reservoirs. Thin, discontinuous cemented zones associated with stylolites occur in the massive, high porosity dolomites of the Upper Smackover at Jay/LEC Field and are the source of horizontal baffles to vertical flow suspected since early days of production and corroborated by full-field reservoir performance studies. It was the need for thin, vertical flow baffles to match historical waterflood arrivals in both full-field and small-area simulation modeling that led to the extensive re-examination of the core and recognition of these previously undetected cemented zones associated with stylolites. Field-wide conventional coring provides a superb core database for describing the physical nature and distribution of stylolites and associated cements. The Smackover interval is cored in over 90% of the wells (149 of the 163 wells). Porosity, permeability and fluid saturations were measured on one-inch core plugs sampled at one-foot intervals. However, small scale heterogeneities such as the reduced permeability associated with the cemented zones above and/or below stylolites were usually not captured using this unbiased core sampling procedure. The cemented zones vary from a few millimeters to several centimeters thick. Probe permeameter analysis has been used to document the decreased permeability adjacent to the stylolites. Three-dimensional geostatistical models of porosity, horizontal matrix permeability and the distribution of stylolites and cemented zones were constructed and used to derive the reservoir properties required for mechanistic simulation models. Reservoir simulations were run on models with and without the cemented zones to (1) determine what impact the cemented zones have on field performance and (2) examine alternative operating strategies for the current miscible nitrogen flood that was initiated in 1981. In the simulations, the cemented zones were assumed to have no effect on porosities and horizontal permeabilities. However, new scaled-up values of vertical permeability were generated to reflect the distribution of cements in the geologic stylolite/cement model. Results show that within the Smackover at Jay/LEC Field, the stylolite-induced baffles enhance oil recovery by reducing gravity segregation and improving the sweep efficiency of the injected nitrogen. Introduction The Upper Jurassic (Oxfordian) Smackover Formation is one of the most prolific hydrocarbon-producing formations in the Gulf Coast region. The producing trend extends in an arcuate pattern around the northern rim of the Gulf of Mexico basin from Texas to Florida. Jay Field, the largest of the Smackover fields, is located in Escambia and Santa Rosa counties, Florida near the eastern border of the Florida Panhandle (Fig. 1). It extends northward into Escambia County, Alabama where it is referred to as Little Escambia Creek (LEC) Field. Jay/LEC Field is approximately 7 miles long and 3 miles wide (Fig. 2). The Smackover Formation is at depths of 15,000 feet (4500 meters) to 16,000 feet (4800 meters) and has an average thickness of about 350 feet (105 meters). Oil is trapped in a northeast-trending anticline on the downthrown side of the Foshee fault. The fault forms the eastern barrier to oil migration and an updip trap to the north is formed by a facies change from porous dolomites to tight limestones and evaporites. Jay/LEC Field was discovered in June 1970 by the Humble St. Regis Paper Co. #1 wildcat drilled six miles south of known production. Jay/LEC Field was unitized and a waterflood using a 3:1 line drive pattern was initiated in 1974 to arrest the rapid pressure decline observed during primary depletion. The current miscible nitrogen flood, based on the water-alternating-gas (WAG) process, was initiated in 1981 to increase reserves and extend the field life. Cumulative oil production from Jay/LEC Field is in excess of 415 million barrels. P. 213^