A Geologic Model For Improved Oil Recovery Project: Tensleep Sandstone, Torchlight Field, Big Horn Basin, Wyoming
Authors: Keith Brian D. , McCaleb James A. 
Published: December 2007

Abstract
A micellar process pilot has been established in the Tensleep reservoir at Torchlight. A detailed geologic model of the reservoir has been constructed to evaluate heterogeneity and boundaries of the reservoir. Laminations impart essentially no heterogeneity whereas anhydrite cement can impose a significant barrier to vertical flow in the reservoir.

Introduction

LOCATION
Torchlight Field is located in the Big Horn Basin in northwestern Wyoming (Fig. 1). The field itself (Fig. 2) is on an anticlinal structure along the east edge of the Big Horn Basin. The structure is several miles out in front of the Big Horn Mountains and is in line with several other structures that occur in the same geologic setting. The field is in Big Horn County.

FIELD HISTORY
Torchlight Field contains numerous reservoir horizons. The earliest drilling was in the early 1900's with the objective being shallow oil from Cretaceous Frontier horizons. Deeper exploration and development was begun in 1947 with the first well completed from the Tensleep Reservoir for an initial flowing potential of 1632 BOPD (see Willingham and Howald, 1965). By the spring of 1949 there were five producing wells from the Tensleep Reservoir. During producing wells from the Tensleep Reservoir. During this time oil production and reservoir pressure in the Tensleep had begun to decline rather rapidly (Fig. 3). All of these wells were then pumped for production into the late 1950's. production into the late 1950's. In 1957, a pilot water injection program was begun and expanded a year later to a larger scale water injection program. Recovery from the secondary waterflood increased to 1650 BOPD from a previous value of 252 BOPD before the flooding (Fig. 3). Increasing water cut in the producing wells in the field caused the water injection program to eventually be discontinued by mid-1967. Production was tapered off and the last Tensleep producing well was shut-in in 1969. With initiation of a five-spot tertiary micellar pilot flood at Torchlight, three wells were drilled in 1975–76 and two previous wells were recompleted in the Tensleep near the center of the field to complete the pattern. At present, the pilot is continuing to undergo waterflooding and pilot is continuing to undergo waterflooding and testing prior to micellar injection. Torchlight Field also produces from the Madison Formation at the present time. A type log from the Lower Phosphoria to the top of the Madison, showing Phosphoria to the top of the Madison, showing the Tensleep Reservoir can be seen in Figure 4.

RESERVOIR DESCRIPTION
The pilot area is the only location of detailed core control in the field. Three of the existing four cores for the Tensleep reservoir were taken for the pilot study. The other core is on the north side of the field, outside the oil-water contact. Thus, a detailed petrologic study of the reservoir can be done using the cores and then expanded to the rest of the Tensleep reservoir in the field using logs. This information is being used to construct various computer simulation models of the reservoir.

LITHOLOGY
Generally, the core is oil stained, but where it is dry and clean the Tensleep is a buff colored, fine-grained (average 0.17mm grain size estimated from thin section) quartz sandstone. X-ray analysis shows only a trace to a maximum of 3% feldspar with no clays present in the rock. The Tensleep sandstone has excellent porosity and permeability within the reservoir. Based on core analysis from the three pilot wells, the average porosity is 14% and the average permeability 112.3 md. Where there is no dolomite or anhydrite cement, the porosity reaches nearly 25% and the permeability over 400 md.

CEMENTATION
Anhydrite is the most prominent cement and the rust abundant.