Combining Bulk Gels and Colloidal Dispersion Gels for Improved Volumetric Sweep Efficiency in a Mature Waterflood

Abstract

Abstract The Golfo San Jorge (GSJ) is located in the southern Argentina provinces of Santa Cruz and Chubut. This hydrocarbon basin occupies a surface area of approximately 170,000 km2, approximately 1/3 of which is offshore. Waterflood oil recovery in many GSJ reservoirs does not exceed 10% OOIP due to the combined effects of reservoir heterogeneity and, in many fields, an adverse mobility ratio. The most significant hydrocarbon accumulations of the basin occur in a series of fluvial and shallow lacustrine reservoirs with significant tuffaceous content. A typical GSJ hydrocarbon reservoir includes a series of relatively thin sandstone packages that are not believed to be naturally fractured. In an effort to distribute water injection more uniformly, most injection wells are equipped with injection mandrels. However, heterogeneity within the productive layers limits the effectiveness of near wellbore selective injection. A recent field pilot combining two types of gel technologies in the Comodoro Rivadavia Formation, one of the most prolific GSJ reservoirs, is encouraging. Based on tracer studies, historical production data and reservoir characterization, approximately 15,000 barrels of MarcitSM gels were injected in each of two adjacent injection wells. Seven months after the Marcit gel treatments, the operator began a colloidal dispersion gel (CDG) pilot in the same two patterns, injecting a total CDG pore volume of approximately 18% in certain layers of the Comodoro Rivadavia Formation. A chemical injection plant was connected at a point upstream of the two injectors so that both wells could be treated simultaneously, with the flexibility to vary the rate and polymer gel concentration in each well. A discussion of the reservoir characterization will be presented as well as the chemical treatment designs, subsequent modifications in the course of the pilot, and recommendations. Introduction Reservoir heterogeneity is the biggest challenge to oil recovery in waterfloods. Heterogenity is essentially any nonuniformity in the productive reservoir, including, but not limited to variablilty in permeability and porosity, anisotrophy, fractures, faults and compartmentalization. Anyone who has studied the classic theories of Buckley and Leverett (1942), Stiles (1949), and Dyktra-Parsons (1950) among others appreciates the dramatic effect of heterogeneity on ultimate oil recovery. In the GSJ of Argentina, injection wells are typically completed with downhole selective injection installations (mandrels) designed to improve vertical distribution of water injection. However, near wellbore mechanical configurations do not address in-depth reservoir heterogeneities. Polymer gels are designed to reduce the effects of reservoir heterogeneity beyond the near wellbore area. The basic premise of any gel technology is that the pre-gel solution, or gelant, will preferentially enter high permeability anomalies responsible for low volumetric sweep efficiency. The theory is that once the gels reduce the flow capacity in the "thief zones", areal and vertical sweep efficiency will be improved. This project describes the pilot area evaluation, gel design and field implementation of two polymer gel technologies: Marcit gels (Sydansk 1938) and colloidal dispersion gels (CDG) (Mack 1994). Marcit gels are high polymer concentration gels designed for application in reservoirs with extreme heterogeneities such as natural or induced fractures, fissures and other multi-darcy permeability anomalies. CDG's are typically large volume, low polymer concentration gels designed to improve sweep efficiency in unfractured matrix reservoirs that exhibit poor waterflood performance. Many waterfloods exhibit both types of heterogeneities.