Solvent Stimulation in Low Gravity Oil Reservoirs

Abstract

Solvent stimulation can continue to be an effective, economical means of enhancing production from low gravity oil reservoirs, provided we understand how it works and are willing to recognize its limitations. Introduction Production from low gravity oil reservoirs, considered Production from low gravity oil reservoirs, considered unfeasible several years ago, has become common practice in many areas today. In California, low practice in many areas today. In California, low gravity crude production (API gravity less than 20 degrees) as a percentage of the state total has increased from 20.7 percent in Jan., 1941, to 31.2 percent in Jan., 1960, and to 51.7 percent in Jan., percent in Jan., 1960, and to 51.7 percent in Jan., 1967. The largest part of this increase is the result of thermal stimulation (mainly steam). For example, from 1966 to 1967 thermal stimulation alone accounted for some 56 percent of the total additional production for that year. The remaining increases are due to a variety of techniques, including improved completion, stimulation (non-thermal), and production methods. A regional shortage of heavy crudes, improved refining capabilities, and rising demand for products such as jet fuel are among the incentives to finding additional means for even further increasing production of such crudes. Solvent stimulation is one of the techniques for enhancing production of low gravity crudes. It involves the uses of low viscosity hydrocarbon solvents in the wellbore, in liners, or in the formation. Possible mechanisms by which increased productivity may Possible mechanisms by which increased productivity may be achieved using solvents include:washing out particulate matter,dissolving viscous deposits, particulate matter,dissolving viscous deposits,diluting the viscous crude, anddemulsifying. Solvent stimulation is not new. Various forms of the technique have been used in California for many years, and it has proven to be an effective, economical stimulation process. However, it is becoming increasingly difficult to apply, principally because the prime candidates have been depleted. If we are to continue to use the technique, we must learn how to apply it more effectively. This can best be accomplished by understanding how solvent stimulation works. Description of the Problem Productivity decline susceptible to solvent stimulation Productivity decline susceptible to solvent stimulation can be caused either by damage or by the nature of the produced fluids. Drilling and completion damage might be from mud or cement filtrate or particle invasion. Production damage could be due to particle migration resulting in gravel pack plugging; scale deposition from produced brines; precipitation of organic matter such as wax and asphaltenes; or viscous emulsions in the formation or liner. Fluid properties likely to adversely affect production are high properties likely to adversely affect production are high viscosity due either to the crude itself or to the presence of water-in-oil emulsions and unstable presence of water-in-oil emulsions and unstable dispersions of asphaltenes or waxes. Typical fields in which solvent stimulation has been applied can be described as having shallow, unconsolidated sands producing viscous crudes and/or emulsions at low pressures and low bottom-hole temperatures. They quite pressures and low bottom-hole temperatures. They quite often have a residual oil content approaching 90 percent of the original oil in place. Production is usually from long intervals 50 to 1,000 ft through slotted liners, with various sand control devices such as gravel prepacks or flow packs. JPT P. 167