Improved Formation Evaluation From Pressure and Conventional Cores Taken With Stable Foam-Bennett Ranch Unit (Wasson Field)
Authors: Hensel W.M. 
Published: December 2007

Improved Formation Evaluation From Pressure and Conventional Cores Pressure and Conventional Cores Taken With Stable Foam-Bennett Ranch Unit (Wasson Field)

The analysis of pressure cores as well as conventional cores taken with stable foam from the San Andres formation in the Bennett Ranch Unit (BRU) of the Wasson field provided a better means of formation evaluation. The pressure-core data enabled correction of conventional-core pressure-core data enabled correction of conventional-core saturations to in-situ conditions. The results of this oil saturation adjustment method corroborate similar procedures cited in the literature. procedures cited in the literature. The pressure at the bit was controlled during the coring operation to minimize the possibility of altering core fluid saturation. Nitrate was used as a tracer material to measure filtrate invasion. The nitrate analyses of the core waters confirmed the lack of filtrate invasion. The foam pressure was adjusted during the coring operation as dictated by surface observations to maintain a balanced condition. Later calculations of these pressures provided a measure of the bottomhole pressure (BHP) provided a measure of the bottomhole pressure (BHP) variations in this thick formation that consists of variable-quality rock. These data provide a qualitative means to evaluate the volumes and paths of injection waters for estimates of the quantity of oil that may be contacted during tertiary recovery operations.

Introduction

Tertiary recovery operations require a large front-end investment in equipment and injectants. The prudent operator will preplan such operations to minimize risk and to optimize the chance for success. The Wasson field of the San Andres formation, located in west Texas is a target for tertiary CO2 recovery projects. Doscher and Wise report that Wasson field has the highest CO2 flooding potential of the 12 Grayburg/San Andres reservoirs listed in their paper. The BRU of the Wasson field may also be a candidate for a tertiary CO2 flood. A pilot area is under consideration. and Sun E and P Co. is pilot area is under consideration. and Sun E and P Co. is attempting to establish a credible data base before implementing such a project. Simplistic material balance approaches (i.e., residual oil equals original oil in place minus volume produced and shrinkage) do not provide the vertical and areal distributions of oil saturation. Log analysis likewise may not provide accurate hydrocarbon saturation information. provide accurate hydrocarbon saturation information. Fluid saturation data from conventional cores seldom yield oil values that are representative of true in-situ values. Pressure coring is a potential tool for providing insight Pressure coring is a potential tool for providing insight into hydrocarbon saturation and distribution. The pressure-core-barrel technique eliminates the reduction in pressure-core-barrel technique eliminates the reduction in oil saturation by gas expansion. However, misleading fluid saturation data may still result if the cores are flushed with drilling mud filtrate during the cutting of the cores. The ideal method is a core cut at balanced BHP conditions and brought to the surface without a reduction in the capture pressure. pressure. If the BHP exceeds the pressure that can be obtained with minimally weighted drilling fluids, a balanced drilling system may be designed. If the BHP is less than that from a normal water gradient, the use of most drilling-fluid systems will result in an overbalanced condition during coring, and pore space flushing will likely occur. Under these BHP conditions, stable foam can be a suitable coring fluid. The foam drilling-fluid system, unlike conventional mud systems, can be modified almost instantaneously to achieve the desired physical properties and pressures required to balance the BHP. pressures required to balance the BHP. Background

The Wasson field is located in Gaines and Yoakum Counties, TX (Fig. 1) and encompasses some 68,500 acres [277 × 106 m2]. Production is from the San Andres dolomite at an average depth of 5,100 to 5,200 ft [1550 to 1600 m]. The producing interval customarily is divided into an upper "first porosity" zone and a deeper zone called "main pay." Discovered in 1936, the field was largely developed by 1941, and waterflood operations started in 1964. Initial oil in place was in excess of 1.5 × 109 bbl [240 × 106 m3]. As of 1974, the remaining reserves were estimated at more than 600 × 106 bbl [95 × 106 m3]. Ghauri and Swift et al. have provided descriptions of the geologic setting and Wasson field core data. The BRU is located in the northernmost tip of the Wasson field (Fig. 1) and contains more than 7,000 productive acres [30 × 106 m2]. The unit was formed in 1964 productive acres [30 × 106 m2]. The unit was formed in 1964 and was operated by the Texas Pacific Oil Co. Inc. Sun acquired the Lower 49 assets of Texas Pacific that included the unit in 1980. Water injection for unitized waterflooding operations began in 1965. As of March 1, 1979, the cumulative oil production was 38.9 × 106 bbl [6.2 × 106 m3]. production was 38.9 × 106 bbl [6.2 × 106 m3]. JPT

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