Cyclic Steam Stimulation in a Tight Clay-Rich Reservoir

Abstract

SPE Members Abstract During the past several years, two proprietary fines stabilizing processes have been field tested in conjunction with cyclic steam stimulation at So. Tapo Canyon Field, Ventura County, California. The Fourth Sespe Formation, the producing zone, is a low-permeability sand (less than 35 md) containing an abundance of clay and non-clay fines. It is characterized by rapid production decline and extreme water sensitivity. Laboratory tests demonstrate the potential for damage by formation fines as well as the effectiveness of the experimental processes in preventing damage. The field tests resulted in widely varying steam responses. This paper describes the differences in steam responses and provides an explanation for some of these differences. Factors that appear to affect steam response include reservoir pressure, changes in oil composition caused by steam injection and heat transfer efficiency. Two wells demonstrated long term stimulation beyond the thermal effects. Overall, the field tests demonstrated that the Sespe can be steam stimulated without damage. Introduction Cyclic steam injection for the purpose of heavy oil recovery can damage water sensitive formations. Well productivity can be impaired by the following effects of steam and water:Clays swell and disperseFines, including non-clay fines, are released by flow of the aqueous phase and migrate to plug pore throatsMinerals are altered at high temperatures to produce fines and soften rockGravel packing media and formation sand dissolve and later precipitate in the formation. In addition to these mechanisms relevant to fluid-sand interactions, production can be impaired by oil-wetting and the deposition of asphaltenes or scale. Other damage mechanisms not specifically related to steam injection, such as mud filtrate invasion or scale deposition, can also affect thermal producers. Production from the Fourth Sespe zone producing at So. Tapo Canyon Field, Ventura County, is especially susceptible to impairment by these mechanisms. Rich in clays and non-clay fines, the Sespe is extremely water-sensitive. Fresh water, used as steam generator feedwater could severely and rapidly reduce permeability. Distilled water, which can contact the formation where steam condenses would also be expected to reduce permeability. In addition, migration of fines with the flow of water can more slowly reduce permeability. Two newly developed processes were applied in these field tests to prevent these potential damage mechanisms. The potential for formation damage is of special concern because formation permeability is low, less than 35 md. The low native permeability contributes to low injectivity and potentially marginal production rates even from undamaged wells. Therefore, damage could reduce the efficiency of heat injection or flow capacity of the well to a level that would be uneconomical. A recent survey shows that over 90% of steam projects involve reservoirs of 500 md or greater. Higher permeability sands can also be water sensitive. P. 105^