Effective Stimulation of Low-Permeability Gas Wells In Western Canada

Abstract

Abstract Rapidly increasing demand for natural gas and recent price incentives halve created renewed interest in the recovery of this product from marginal and previously non-commercial reservoirs as well as obtaining better productivity from all gas reservoirs. Many of the low-permeability gas sands of Western Canada could become more attractive investments if successful and efficient stimulation could provide profitable rates of return. This paper discusses the approach taken to evaluate the different low-permeability formations as to the type of frac acid stimulation to use, the new fluids developed specifically for this type of problem and field stimulation results for the different fluids used Introduction INCREASING worldwide demand for natural gas and resulting shortages in many areas have created an intense interest in the search for and production of this product. Intensive exploration is underway throughout the world for new gas reservoirs in ultra-deep formations and in extremely remote locations. Other new sources of gas, which are receiving renewed interest are the low-permeability gas reservoirs, which have been marginal or non-commercial in the past. The reason for their marginal or non-commercial status has been the inability to successfully stimulate them to provide flow rates which were adequate at existing prices. More favorable prices have now reduced the flow rates required for profitable production, but stimulation is still necessary' and the stimulation problems still exist. Many such reservoirs exist in Western Canada. The problems involved in stimulation are the same as those involved in most areas. This paper will discuss these problems and show how a careful evaluation of the reservoir properties can lead to a treatment design which will provide economical and successful stimulation. Formation Damage Failure of low-permeability gas wells to respond to stimulation is generally the result of formation damage incurred during the stimulation process. Recent papers have discussed this damage from the standpoint of two causes: Water Sensitivity and Liquid Retention. WATER SENSITIVITY Many formations suffer a reduction in permeability when contacted by water or water-base fluids. These formations are said to be water sensitive. The permeability reduction in such formations is caused by blockage of pore channels resulting from dispersal and migration of clay particles. Invading water-base fluids can cause changes in the ionic composition of the interstitial aqueous phase. This change causes clay particles in the formation to disperse and become entrained in the moving fluid. They then form microscopic filter cakes on the narrow pore openings and restrict fluid flow through the formation. More complete discussions of clay chemistry and dispersion are available(1,2), and the subject will not be further discussed here. The fact of importance is that formation damage from migratory fines is generally induced by the choice of fluids(J,4). Figure 1 shows the magnitude of damage that can result from the use of improper fluids. The data shown were obtained by making permeability measurements while flowing a sequence of fluids at constant pressure through a Berea sandstone core.