The Paleocene/Eocene age First Eocene dolomite reservoir is estimated to contain than 10 billion barrels of oil of which only a small percentage will be produced during primary development. Consequently, steam flooding is being investigated as an appropriate EOR option. A 1.25-acre, single pattern pilot (SST) and a 40-acre, 16 pattern pilot (LSP) are in progress. The detailed pilot area log, core, and seismic data provide a unique opportunity to assess reservoir heterogeneity. Analysis of temperature and petrophysical logs obtained in a temperature observation well located 35 feet from the SST injector show that a vertical barrier to steam migration exists. Two, relatively thick, very low porosity and very low permeability nodular evaporite-rich zones that were predicted to be the most likely barriers do not appear to be a vertical barrier. Instead, an interval characterized by numerous thin, cycle caps, characterized by muddy, finely crystalline dolomites interpreted to be tidal flat facies may be the vertical barrier. Each of these cycle caps also exhibit signs of subaerial exposure which may also contribute to the generally low porosity and very low permeability of the cycle caps. Detailed studies, including micro-permeameter measurements, quantitative mineralogical studies, and micro-CT scans were used to further characterize this interval. The geological assessments of heterogeneity are supplemented by a history-matched simulation model that suggests the evaporite-rich zones may have acted as short term baffles but that the vertical barrier to steam migration is coincident with the interval with abundant tidal flat cycle caps and exposure surfaces. Geological and other reservoir data obtained from the LSP suggest that similar vertical barriers may exist in the pilot area. Early steamflooding results show a very positive response to steam injection as well as multiple thermal "events" (most likely baffles rather than barriers) in the lowermost flooded zones at the LSP. The LSP data allows inferences to be made regarding the occurrence and distribution of lateral high permeability "connections" between injectors and producers as well as the overall reservoir response to steam injection. While the rapid temperature response observed in a few wells may reflect localized fractures or karst-like zones, numerical simulation using very fine grids (1.25 m cell size) shows that some of the LSP wells may experience very short breakthrough times without the need for fracture or karst-like zones.
The oil and gas industry uses static and dynamic reservoir models to generate production forecasts. Generally, industry look-backs (and informal networking) have shown that forecasts are often optimistic for both "Greenfield" projects with limited data and "Brownfield" projects with abundant data. One of the main sources of optimistic forecasts is biased estimates of the original or net/targeted in place hydrocarbon volume. Some bias is due to sampling, particularly for Greenfield developments, and this bias can be reduced statistically or by use of appropriate uncertainty-based workflows together with a reasonable uncertainty assessment that includes the available data and an appropriate suite of analogs. An underappreciated additional source of significant bias related to in place volumes is the use of a stochastic reservoir property model to locate wells. The use of stochastic earth models combined with well placement optimization workflows is likely to yield significantly optimistic forecasts. Well placement and optimization should be based on property distributions derived via estimation methods such as kriging rather than stochastic methods.Reservoir models are usually generated using sophisticated software. Elegant geological models can be generated without an adequate understanding of the limitations imposed by the available data, associated uncertainty, or the underlying stochastic algorithms and their input requirements (e.g. the semivariogram; a measure of heterogeneity). For example, forecasts based on models generated using different semivariogram ranges (all other input parameters held constant) show that the recovery factor for waterflooding may be impacted significantly whereas for steamflooding the impact may be negligible. Recent studies using an extensively sampled portion of a heavy oil carbonate reservoir have shown that grid size, which has minimal effect on primary recovery forecasts, will impact forecasts for displacement processes. Generally, if there are fewer than ten cells between injectors and producers in dynamic models, waterflood forecasts and perhaps also steamflood forecasts will be optimistic. The impact of static and dynamic model parameter choices on forecast bias should be evaluated as part of any comprehensive reservoir study.
The Paleocene/Eocene First Eocene dolomite reservoir is a candidate for continuous steamflooding due to its large resource base and low estimated primary recovery. There are two steamflood pilot projects in operation to evaluate reservoir response to steam injection: a single pattern pilot (SST) and a 40-acre, 16 pattern pilot (LSP). At the SST an interval with abundant tidal flat cycle caps characterized by muddy, finely crystalline dolomites with low porosity and permeability may be the observed vertical barrier to steam migration. Detailed studies, including micropermeameter measurements and micro-CT scans were used to characterize the heterogeneity. Data suggest that similar vertical barriers may exist at the LSP. Early steamflooding results show a positive response to injection and multiple thermal events (likely baffles rather than barriers). The data also shows the occurrence and distribution of some lateral high permeability pathways between injectors and producers as well as between producers. While the rapid temperature response observed in a few wells may reflect fractures or karst-like zones, simulation using very fine grids shows that some wells will experience very short breakthrough times without fracture or karst-like zones. Injection of high temperature, high pH fluids may induce fluid/rock interactions that affect reservoir fluid flow near-well and in-depth. This in turn could affect storage capacity, production and injectivity. Reactive transport models (2D-RTM) were run to simulate high pH steam injection into the First Eocene reservoir for a continuous injection period of 6-12 months to understand possible changes in mineralogy, coupled with porosity change and potential scaling. Initial results predict precipitation of calcite and brucite, dissolution of dolomite and anhydrite, and conversion of gypsum to anhydrite. Sensitivity studies examined the impact of steam quality, pH, rock surface area, reaction rates, and mineralogy.
Since 1956 the Wafra 1 st Eocene Reservoir in Kuwait has significant heavy oil from dolomitized peloidal grainstones and packstones/wackestones interlayered by nodular to tabular evaporites. As primary recovery will likely be low, options are being evaluating for a possible full field steamflood project to improve oil recovery. As part of this evaluation a multidisciplinary team was formed in January 2009 to help provide a detailed and data driven view of the historical performance of the reservoir. LSP N Figure 2. Structure map for top of the First Eocene reservoir shows the location of the two steamflood pilots. SST = single 1.25-acre 5 spot pattern pilot; LSP = 16, 2.5-acre pilot 5 spot pattern pilot. The SST is located in the Main Area of the reservoir and the LSP is located in the West (or Southwest) Area of the reservoir. After Meddaugh et al (2011).
Reservoir forecasts tend to be optimistic. Forecasts for IOR/EOR projects tend to be particularly optimistic. Sources of the optimism can be divided into several broad categories including: Dataquantity, quality, sampling bias; Static Modeling-model complexity, particularly for permeability contrasts; Model parameter/algorithms choice; and, Dynamic Modeling-model detail/complexity, upscaling, well location optimization. In addition, human factors also tend to drive projects towards optimistic forecasts. Based studies of a number of reservoirs representing a variety of lithology types and depositional environments with data densities ranging from low (greenfield) to extremely high (multipattern pilots) observations on modeling and forecast accuracy can be made relative to IOR/EOR forecast results, in particular. Among the most critical modeling parameters are the areal grid size and the semivariogram range parameter. Optimistic estimates of the in place hydrocarbon volume is also one of the most significant sources of optimistic forecasts. Some of this latter bias is due to sampling, particularly for green-field developments, and some due to inappropriate use of analogs. This bias can be reduced with uncertainty-based analyses and workflows and an appropriate suite of analogs. Well location optimization based on stochastic models is an under-appreciated source of forecast optimism.
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