Small Propane Slug Proving Success in Slaughter Field Lease
Authors: Sessions Roger E. 
Published: December 2007

Abstract
A propane-gas-water miscible-phase displacement process has been in operation on the H. T. Boyd lease, Slaughter field, since May, 1958. The total requirement of 255,416 bbl of liquid propane and 2,625 MMcf of residue gas has been injected. These materials were injected to obtain the high unit displacement efficiency of a miscible process in a portion of the reservoir. This lease is not bounded and the pressure differential created by high injection rates places a limit on the area to be miscibly swept. Water injection is being used to improve the miscible sweep, to displace the oil bank, and to flood portions of the reservoir not miscibly contacted. A total of 4.2-million bbl of water has been injected. The lease performance history indicates the small propane slugs have created a substantial oil bank in the major portion of the reservoir. Considerable test data have been obtained and interpreted to determine the type of displacement in each injection area. These data show that a portion of each injection area has been miscibly swept. There is a wide variation in performance between the areas, and this difference is attributed to permeability stratification. Field data have been used to make required changes in the original plan of operation, and miscible displacement is continuing. The small propane slugs (about 3 per cent of the volume to be miscibly contacted) are expected to sweep at least 18 per cent of the hydrocarbon pore volume. The predicted recovery for this project is 58 per cent of the oil in place. This recovery is about 1.5-million bbl of oil over the predicted waterflood recovery, and it is almost three times the predicted primary recovery.

Introduction
A combination secondary recovery process has been in operation on the H. T. Boyd lease, Slaughter (San Andres) field, Tex., since May 9, 1958. A propane-gas-water miscible-phase displacement process to be followed by water flooding is being used to increase the ultimate recovery from this lease. The project was designed to take advantage of the desirable features of both processes. The past performance of this project has answered some of the questions concerning the application of the gas-miscible slug process. Considerable test data have been obtained and interpreted to learn bow effective the small propane slugs injected here have been in displacing oil miscibly and to aid in developing plans for the future operation of the lease.

Plan of Operation
The original plan of operation can be described in terms of the type of fluid injected and the type of displacement planned. Step 1-Inject a volume of propane into each injection area to miscibly sweep a portion of the hydrocarbon pore volume. The liquid propane is miscible with the reservoir fluid and efficiently displaces it from the rock. Step 2-Inject a volume of lean hydrocarbon gas, equal to the volume left as residual gas to water flooding in the volume to be swept miscibly. A gas slug is injected to maintain a buffer zone between the propane and the water zone. The gas must be injected at a rate high enough to maintain the miscible-displacement pressure at the gas-propane front. The miscible-displacement pressure for this project is about 1,300 psia. The gas will then miscibly displace the propane slug. Step 3-Inject water at a high pressure to maintain the pressure of the gas-propane boundary at, or above, the miscible-displacement pressure until the planned sweep by miscible displacement is achieved. The water immiscibly displaces the gas slug and increases the sweep efficiency of the miscible phase. Step 4-Continue to inject water to abandonment, but at a lower pressure. This lease is surrounded by other producing wells and the pressure of the gas-propane front must be maintained above 1,300 psia. This is several hundred pounds greater than the offset lease pressures. This limits the area to be swept miscibly by propane-gas-water injection to the central portion of the lease. After the miscible sweep has been completed, water flooding will be used to increase the recovery from the portion of the lease not contacted by the miscible sweep. The area to be swept by water will depend upon the extent of the offset lease cooperation.

Geology and Reservoir Data
The Slaughter field is one of several large San Andres reservoirs in the Permian Basin.
JPT
P. 31^