Eight attempts have been made to drill and complete horizontal wells in the Spraberry trend of West Texas. The results have been disappointing to date. The production history of each attempt will be discussed, along with an economic analysis. By using the drainhole production and results from vertical well completions, a model for predicting the performance of these wells will be proposed. The key input to the model is a modified Coates- Denoo relationship between porosity and permeability that utilizes openhole wireline inputs. The model predictions will then be compared to actual performance and used to help predict the optimum completion performance and used to help predict the optimum completion strategy for wells in the Spraberry trend.
The Spraberry trend of West Texas has been the target of extensive drilling since its discovery in 1949. Early wells in the trend were characterized by high initial flow rates after small stimulation treatments. The average initial potential from one study of 718 wells was 318 BOPD from open hole completions in the Upper Spraberry. The high initial potentials had sparked a boom in the early 1950's, where up to 203 rigs were running by mid 1951. The reservoir extent seemed enormous, with production established over an area 150 miles long and 50 miles wide. Operators were convinced they had found "a potentially great source of oil reserves.."
This boom was short lived, as the high potentials were accompanied by rapid declines. In three cases, wells produced from 26,000 to 40,000 BO in 15 to 17 months prior to abandonment. The poor success in sustaining economic production rates led operators to try different techniques to improve productivity. The two major techniques employed by operators were hydraulic fracturing and waterflooding.
The earliest wells were open hole completed with a small cleanup "hydrafrac". A survey of 781 wells conducted in 1952 indicated that 92% of the wells in the survey were openhole completed, with the majority of the treatments consisted of 1500 gal of fluid or less. As operators gained experience, the treatments increased to 4500 gal of oil based fluid carrying 2400 lb of 20/40 sand. The treatments were generally pumped down 2-3/8 in. tubing at an average injection rate of 5 bbl/min.
These early treatments did not limit the steep declines, and by 1954 operators began to implement treatments with higher rates and volumes. In a series of re-fracture treatments documented by Gerolde there were substantial improvements in productivity with larger job sizes. A typical re-fracture treatment consisted of 21,000 gal of frac fluid carrying 31,500 lb of 20/40 sand at 42-56 bbl/min down casing. Several wells that were producing less than 10 BOPD were increased to over 100 BOPD. One well recovered 55,438 BO in 34 months after a 4500 gal fracture treatment. Prior to the re-fracture treatment the well was producing 1 BOPD. Following a re-fracture treatment consisting of 21,000 gal oil based fluid and 31,500 lb 20/40 sand the daily production rate increased to 333 BOPD and the well recovered an additional 34,155 BO in the next 18 months.
In the period following the early 50's, waterflooding gained popularity as a means to improve recoveries. The declines in production rates were accompanied by rapid declines in reservoir pressure, and waterflooding was considered as a technique to reverse this decline in pressure and to sustain production. On 40 acre spacing the reservoir pressures declined 3.86 psi per day, while on 80 acre spacing the reservoir pressures declined 0.59 psi per day. Brownscombe and Dye (1952) suggested that oil could be displaced in the Spraberry by capillary imbibition of water into the rock. By 1953 it was suggested by Elkins that the low permeability matrix pores held the majority of the oil, and that the natural fractures provided flow channels for the oil to reach the wellbore. The concept of provided flow channels for the oil to reach the wellbore. The concept of waterflooding the Spraberry was proposed to help limit the decline in reservoir pressure and to displace the oil in the matrix.