This paper presents the results of an extensive study and field test carried out at the site of Prudhoe Bay's four oily waste injection wells. Prudhoe Bay's four oily waste injection wells. The field work was part of an overall environmental assessment intended to:
confirm earlier results indicating that no fluid communication was occurring with the permafrost;
determine optimum conditions for the disposal of waste in the presence of hydraulically-induced fractures;
substantiate that an increased injection pressure could be safely implemented.
A three day injection test, including a step-rate stage, was carried out. Data collected included surface and downhole pressures, in-situ stress measurements, and pressures, in-situ stress measurements, and monitoring of ground surface deflections and wellbore hydraulic impedance.
Radially symmetric surface tilt patterns showed that the test well connects to a horizontal fracture of 60-foot radius. Wellbore impedance indicated that a horizontal fracture with 9–18 foot radius communicated with the well. Integration of rock mechanics, historical information, and the collected data provided a clear picture of what was occurring underground. The different evaluation techniques showed consistent results as reflected in the estimated fracture size, placement and damage zone properties. properties.
In 1973, five wells drilled to a 2200-foot depth near Drill Site 6, Prudhoe Bay Field, formed a five spot pattern with a well spacing of 23 feet. They were used for an extensive thaw subsidence test, then shut-in. They were later converted to injection for waste fluid disposal under a Class II UIC permit. The Center well was never perforated and the Northeast well has been abandoned. Fluids can be directed to any one of the remaining three. Each well has 20–50 feet of perforations below the permafrost at 2000 feet. Injection is intermittent, depending upon when trucked fluids arrive at the site. The plant typically operates at a 900 psi discharge pressure which means the rate usually varies between 1–2 bpm, depending upon fluid characteristics. Approximately 2 million barrels have been injected. Disposal rates average 500 bpd with peak rates of 4000 bpd.
A simple description of the injection stream is difficult because of the many sources. Predominantly it consists of waste waters but includes contaminated crude oil, vessel sludge, acids, unused frac sand, gels, drilling muds, stimulation fluids and formation fines, unset cement, tank bottoms, and solutions of methanol and glycols. The range in temperatures, viscosities, and densities is large. The solids content is sometimes very high.
Figure 1 shows a typical gamma ray log of the injection interval. The wastes are injected into a formation composed of thick sandstone and gravel intervals interbedded with shale, siltstone and mudstone. The wells are perforated in a heterogeneous interval of thinly perforated in a heterogeneous interval of thinly bedded shales, siltstones and sandstones, 150 feet below the permafrost. The interval occurs 30 feet below a laterally continuous thick bedded sandstone with excellent porosity and permeability. This sand rises toward the permeability. This sand rises toward the southwest and eventually intersects the permafrost.