Actual Performance Compared to a 20 Year Old Probabilistic Reserve Estimate
Authors: Wright John D. 
Published: December 2007

The Trap Spring oil field in Nye County, Nevada was discovered over twenty years ago. Early in the life of the field a probabilistic estimate of reserves was performed based on the volumetric equation. Actual production history from the wells that were evaluated is compared to the results of the probabilistic estimate. Reserve estimates for the individual wells did not bracket truth in many cases, but the overall reserves for the total field were surprisingly close to the estimate.

Introduction

The Trap Spring oil field in east central Nevada was the second oil field discovered in the state. It lies about seven miles west of the first oil field in the state - the Eagle Springs field. (Fig. 1)

Originally drilled on a seismic high to test the Paleozoic, Eagle Pass (limestone) formation, the Trap Spring #1 recovered 3914 feet of oil in October, 1976 on a drill stem test of the Tertiary volcanics. Development drilling followed rapidly and it was learned that the Trap Spring field was a structural trap dipping to the east and bounded up-structure by a fault and downstructure by water. The pay zone is a fractured, welded, volcanic tuff of Tertiary age deposited as an ash flow. The rock does not appear to have any hydrocarbon-saturated matrix pore volume. All of the oil is contained in fractures created during the cooling of the ash flow or during subsequent faulting. The tuff zone ranges in thickness from 400 to 2500 feet.

The oil is a black, viscous, moderately high pour point crude with no dissolved gas. The gravity ranges from 20 to 26 API; the viscosity is around 100 cp, and the pour point is around 20 F. The reservoir energy is supplied by an active water drive from the downdip aquifer.

Many of the wells are completed open-hole, with 7-inch casing set in the top of the zone. Most of the wells were plugged back to depths between 4000 and 5000 feet. On the better wells no stimulation was performed while poorer wells had 15,000 to 100,000 pound frac jobs. The original reservoir pressure was very close to hydrostatic. This, coupled with the extremely high permeabilities (1+ darcies) caused the wells to flow initially. If the flowing rate was not high enough, the well was pumped with sucker rod systems. Fluid levels ranged from 100' or less below the surface to 4000'. On most of the better wells, the fluid level was 1000' or less.

The development of the area has continued to the north of the original wells with the drilling of the Munson Ranch wells beginning in late 1978. These wells were not included in the study discussed here. Ref. 1 contains an excellent discussion of the geology of the field.

A reserve report was prepared using Monte Carlo simulation in 1978 after 18 months of production and was updated after 30 months of production. A comparison of the results of that report with the actual production history over the last 20 years follows.

Method of Analysis

The objectives of the reserve report were simply to estimate ultimate recovery and remaining reserves. No attempt was made to forecast future production rates or perform economic analysis.

Three of the classic methods of determining reserves were found to be inapplicable. The analogy method could not be used, because, at the time, there were no analogous fields. (Eagle Springs produced out of two limestone zones in addition to the volcanics.) The material balance technique was not appropriate because the strong water drive was providing almost total pressure support. In addition, the results would be extremely sensitive to the fluid compressibility and the unknown rock compressibility because the reservoir pressure was considerably above the bubble point pressure. P. 529^