CSAMT Mapping of a Utah Tar Sand Steamflood
Authors: Wayland J.R. , Lee D.O. , Cabe T.J. 
Published: December 2007

The mapping of thermal fronts associated with EOR techniques from the surface has important application to improved reservoir stimulation. Real-time knowledge of how a thermal EOR front is developing would help an operator to optimize production well placement as well as to take corrective steps to maximize production. The surface mapping technique that we are evaluating is the controlled-source audio frequency magnetotelluric (CSAMT) electromagnetic (EM) induction geophysical prospecting technique. prospecting technique. The analysis of the field data, using results from laboratory studies, is described. Normalized maps show the location of the steam front that is in agreement with production data, temperature data from monitoring wells, and posttest coring data. The location of a major bypass of steam into an underlying formation was also mapped. The results indicate that the CSAMT technique shows promise for monitoring steam-front locations.

Introduction

A steamflood experiment. designated TS- 1S, was conducted by the U.S. DOE Laramie Energy Technology Center (LETC) in the Northwest Asphalt Ridge tar sand. The later stages of this experiment were mapped at depth with the CSAMT technique to assist in determining the location of the steam front. The CSAMT equipment produces an induced EM field (EMF) in the earth from a transmitting antenna. The interaction of this EMF with subsurface formations, including the pay zone where the EOR processes are applied, is indicated in the measured electric (E) and magnetic (H) fields at surface observation points above the zone of interest. The basic assumption of the CSAMT technique is that significant electrical property changes occur in the reservoir as the result of the steamflood. The mapping of EOR processes can be useful for improving reservoir stimulation and recovery economics. Knowledge of the location of the steam front would be helpful in determining the location of new production or injection wells. In addition, action based on this knowledge could be taken to correct potential bypass situations. Thus a scoping study was performed on a steamflood experiment being conducted by the U.S. DOE LETC. Steam injection began on April 23, 1980, for the first steamflood experiment in the Northwest Asphalt Ridge tar sand (oil sand) near Vernal. UT. 1 Steam was injected into the center well of two concentric inverted five-spot patterns of 0. I and 0. 25 acres 1405 and 10 1 2 m 2 i each (Fig. 1). The Rimrock sandstone member of the Cretaceous Mesaverde formation on Northwest Asphalt Ridge is highly saturated, semiconsolidated, fine-grained, and interrupted by many low-saturation siltstone or shale intervals. The tar sand sections vary in thickness from about 1 to more than 40 ft [0. 3 to more than 12m]. The Rimrock formation outcrops about 0.25 mile [0.4 km] north-east of the test site and dips to the southwest at about 20 degrees. Adjacent to the test area, the Rimrock member varies in thickness from 150 to about 200 ft 146 to 61 m]. There is major faulting in the area, with the possibility of undetected minor faulting also being a major concern. The experimental zone is a fairly continuous tar sand interval 45 ft 114 m] thick starting at 500 ft [152 m] and located in the lower half of the Rimrock. It is bounded on the bottom by a thin (10-ft [3-m]) layer of shale underlain by another tar sand zone. Core data show that 90% of the zone averaged 10-md gas permeability with the oil in place, and the bottom 10% had several darcies of permeability. This lower interval appeared to be fractured permeability. This lower interval appeared to be fractured and semiconsolidated. The pay zone contained a 30 degrees API [0.876 kg/cm ] bitumen with a viscosity greater than 106 Cp [103 Pa - s] at reservoir conditions and average oil saturation of 79% PV. The average core porosity was 28%. In the 160 days of operation until Sept. 29, 1980, a total of- 65 × 10 3 bbl [10.4 × 10 3 m 3] of cold-water-equivalent steam were injected. During the course of the experiment, 1,150 bbl [183 m3] of oil and 6,250 bbl 1994 m3] of water were produced. The best injection-well/production- well communication was the downdip to Well 3P6, which occurred within 4 hours as compared with the several days required for response from the updip to Wells 3P4, 3P5, and 3P8. The behavior of the producing and temperature- monitoring wells is summarized in Table 1. It appears that the steam front advanced first along the strike line (northwest to southeast) of the pattern (roughly through Wells 3M1 and 3M3). The first temperature increases occurred in the bottom 5 ft [1.5 m] within 67 hours. Steam break-through occurred at Well 3P6 within the first month of operation. By July 2, 1980 (Day 65). Well 3P8 had steam breakthrough. For the remainder of the experiment the steam probably moved through the downdip segment of the pattern.

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