Pilot Testing Issues of Chemical EOR in Large Fractured Carbonate Reservoirs
Many world class large carbonate reservoirs leave behind at least half of the initial oil in place. Typically water injection is used to improve oil recovery while gas injection is used to maintain pressure or to promote oil gravity drainage. Immiscible gas injection, including injection of CO2, has been considered but not implemented on a large scale for economic reasons. Furthermore, interest in using surfactants in large carbonate reservoirs has recently flourished. As a result, we began to investigate the viability of designing and conducting a manageable pilot test program in a large fractured carbonate reservoir using a single-well, dual-completion system to evaluate the efficacy of the surfactant oil mobilization and oil capture. However, pilot testing in large reservoirs is very expensive and requires a long time to complete. These issues are less problematic in pilot testing of small and thin reservoirs in onshore field. In this paper we will present the results of a conceptual model to simulate the performance of surfactant flooding in the above-mentioned pilot test configuration. Three different model formulations, having different approaches to gridding and grid-refinement, were used. These include conventional dual-porosity, dual-permeability, and single-porosity models with variable porosity and permeability to simulate fracture-matrix interactions. Simulation of pilot tests using dual-porosity models shows that gravity is most effective during waterflood but not as effective during the surfactant injection while in the dual-permeability models, the surfactant oil recovery is greater because both gravity and viscous displacement contribute. We will explain the reasons and will indicate which model is more reliable. In general the results of this study give an insight into the viability of using surfactant injection in thick carbonate reservoirs both in the pilot and production stage. Introduction In the U.S, typically about a third of the original oil in place (OOIP) is recovered by primary and secondary recovery processes, leaving two-thirds of the oil behind as remaining oil (NPC, 1984). About 60% of world‟s discovered oil reserves are in carbonate reservoirs, and many of these reservoirs are naturally fractured (Rohel and Choquette, 1985). According to a recent review of 100 fractured reservoirs (Allan and Sun, 2003), fractured carbonate reservoirs with high matrix porosity and low matrix permeability could be good candidates for enhanced oil recovery (EOR) processes. The oil recovery from these reservoirs is typically very low because about 80% of fractured carbonate reservoirs are either oil-wet or mixed-wet. Injected water will not penetrate easily into the oil-wet porous matrix to displace oil. Wettability of carbonate reservoirs probably is the most important oil recovery controlling parameter (Morrow and Mason, 2001; Tong et al., 2002; Hirasaki and Zhang, 2006). Typically water injection is used to improve oil recovery, while gas injection is used to maintain pressure or to promote oil gravity drainage as an IOR process. If gas injection is miscible or near-miscible, oil recovery is enhanced because a fraction of the conventional residual oil is mobilized by miscibility or near-miscibility conditions. Water and gas injection have been used to produce oil from the matrix in naturally fractured reservoirs (NFR) mainly by gravity drainage. Viscous displacement in fracture-dominated NFR generally plays a minor role except for chemical flooding, where surfactants might enter the matrix from fractures with assistance from viscous displacement to mobilize oil. Even this effect appears to be small because of the lack of deep surfactant penetration. In water-wet NFR, water imbibes strongly into the matrix and produces a lot of oil. However, in oil-wet reservoirs, water-flooding is relatively inefficient. This is characterized by the early water breakthrough and rapidly increasing water-oil ratio. The reason is that, for an oil-wet reservoir, the injected water tends to travel only through the fractures and not enter the
Mechanically robust re-crosslinkable polymeric hydrogels for water management of void space conduits containing reservoirs
Abstract A unique preformed particle gel with re-crosslinkable ability to re-form bulk gel for chemical enhanced oil recovery of void space conduits containing reservoir is described herein. The formation of the bulk gel was achieved by reversible ionic crosslinks of the preformed particle gels as induced by brine under reservoir conditions. The re-crosslinked hydrogel with ionic features showed rubber-like mechanical behaviors, a critical parameter of the particle gels to increase plugging efficiency. The re-crosslinking ability of the preformed particle gels under different salinity could be controlled by tuning the ionic interactions within the hydrogel. In addition, the long-term thermal stability of the re-crosslinked bulk gel has been demonstrated. Furthermore, the re-crosslinked bulk gels could significantly reduce permeability of large fractures as demonstrated by core flooding experiments compared with traditional preformed particle gels. The re-crosslinkable bulk gel with robust mechanical properties could offer the candidate to block void space conduits containing reservoirs for enhanced oil recovery.
The Redevelopment of a Mature Waterflood Through the Application of Horizontal Multilateral Drilling Technology in Preparation for Tertiary CO2 WAG Operations
Multi-lateral horizontal drilling technology has been employed since late 1996 in the mature carbonate waterflood of the Ratherford Unit, Greater Aneth Field, San Juan County, Utah, in preparation for the implementation of a CO 2 Water-Alternating-Gas (WAG) tertiary recovery project, planned for initiation in 2000. The program of multi-lateral reentries of 21 existing producers and 24 existing injectors over a two- year period (late-1996 through late-1998) was comprised of 100 laterals targeting the highly-stratified layers of the Upper Desert Creek, Miscible flooding of the low-permeability Pennsylvanian Upper Desert Creek Zone I has been shown by analysis of the adjacent McElmo Creek Unit to be economic only through increased vertical/areal sweep efficiencies and processing rate enhancements obtained by the application of horizontal injector-producer pairs. The results of this study were confirmed by the 1994 -1995 Ratherford Unit horizontal injection well drilling program, the 1996 Ratherford Unit multi-lateral production well drilling program, and the 1995 horizontal CO 2 pilot pattern in the adjacent McElmo Creek Unit. This paper will review the basic geologic and engineering concepts behind the application of horizontal drilling technology within the Ratherford Unit, detail the multi-lateral techniques employed and process improvements made through time, and summarize the actual field results.