Evaluation of Cyclic Steam Operations at Cymric 1Y Diatomite

Abstract

Abstract Cymric 1Y is a heavy oil diatomite reservoir in California, U.S.A. with an estimated original oil-in-place (OOIP) of 425 MMSTB. Diatomite reservoirs have low permeability (average of 5 md or less) and high porosity (50–60%). The primary objective of this study was to investigate future development options using a consistent Base case for the cyclic operation in the 1Y pilot area. Field data from the in-fill pilot area, with a well spacing of 5/8-acre, and a fine-grid simulation of a four-well segment was used in this study. Results show that fine grids (of the order of one ft), perpendicular to the steam-induced fracture orientation, are necessary to properly capture the effects of sharp temperature and saturation gradients in the primary fluid flow direction. Initial saturation, reservoir matrix permeability, fracture half-length, capillary pressure, and flowing bottom-hole pressure were identified as key parameters at Cymric 1Y. Results also show that the cyclic steam development at 5/8-acre may result in 22% recovery. Infilling to 5/16-acre spacing could increase the recovery to 34% OOIP. Success of steam drive at Cymric 1Y would require that fractures remain open at both injectors and producers. A successful steam drive could further boost the ultimate recovery. Background Production at Cymric 1Y occurs from the upper portion of the upper Miocene Antelope Shale of the Monterey Group. Exploitation strategy for this reservoir is based on short steam cycles with steam injection above fracture pressure for 3–6 days. Steam injection causes a vertical fracture to develop at the well. After steam injection, well is allowed to soak for 3–4 days. During the soak period, steam and condensed hot water further propagate into the reservoir. Well is then allowed to produce under self-flowing conditions for 15–25 days. Viscosity reduction and capillary imbibition are the primary recovery mechanisms for Cymric 1Y. Also, steam-assisted lift occurs during the production cycle. A typical cycle lasts for about a month or less. Currently, Cymric 1Y produces at over 20,000 STB/day with a steam/oil ratio of 2–2.5. In the past, a single-well, layer-cake, cyclic steam model for Cymric 1Y diatomite reservoir operating under fracturing conditions was developed by Kumar and Beatty1. This paper outlined most of the rock and fluid parameters that have been used for all subsequent Cymric 1Y simulation studies at Chevron, including this study. This paper also established the need for fine gridblocks in the direction perpendicular to the fracture to capture the effects of sharp saturation and temperature gradients that occur away from the fracture face in Cymric 1Y cyclic steam operation. Using a detailed geostatistical model of the pilot area incorporating core and log data from 63 wells, Fong et al.2,3 describe the development of a coarse-grid reservoir simulation model for four patterns surrounding wells 2112R, 2212S, 2113S and 2213S (Fig. 1). This model covered 11 wells in the pilot area and was intended to develop an understanding of well-to-well interference effects for long-term operations under cyclic steam or steamdrive operations. This model will be referred to as the Coarse Grid Model (CGM) throughout this paper. To address competing requirements for fine grids and multi-well models, this study has used the CGM geostatistical model to extract a reservoir simulation model for a four-well segment within the pilot area. The current model uses fine grids away from the fracture to capture the effects of sharp temperature and saturation gradients. This model also allows for an infill well. Thus, we can study well-to-well interference effects, and future cyclic steam and steam drive scenarios using the model presented in this study.