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This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 156163, "Integrated Dynamic-Flow Analysis To Characterize an Unconventional Reservoir in Argentina: The Loma La Lata Case," by Matias Fernandez Badessich and Vicente Berrios, YPF, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 8-10 October. The paper has not been peer reviewed.
In November 2010, YPF brought the first shale-oil well on line in Argentina in the Loma La Lata field after fracturing the Vaca Muerta formation, the main source rock in the Neuquen basin. Production forecasting and reserves estimation in this kind of reservoir are fraught with challenges. Several reservoir- and production-analysis techniques were applied, including pressure-transient analysis (PTA), rate-transient analysis (RTA), interpretation of available diagnostic-fracture-injectivity tests (DFITs), and time-lapsed production logging. YPF implemented a workflow to analyze dynamic data that capture the physics of the flow process, to explain the observed data, and to forecast reserves for a range of assumed in-place volumes.
The formation in this field has a thickness range of 25 m in the proximal areas to 450 m at the basin center. Formation depth ranges from less than 1000 m at the basin margin to 4000 m near the basin center. With the wide areal distribution and variable thickness and over-burden, the Vaca Muerta has produced low-gas/oil-ratio liquid hydrocarbons, volatile oils, gas with condensate, and dry gas.
According to log and core analysis, the matrix porosity of the Vaca Muerta shale varies from 4 to 14%, with an average of 9%, while matrix permeabilities span from hundreds of nanodarcies to production rates. At the time this paper was written, 27 vertical wells and three horizontal wells in this field had been placed on production.
Most of the wells required massive hydraulic fractures to achieve commercial rates. Typically, four fracture stages were performed in the vertical wells, while 10 stages were completed in the approximately 1000-m-long horizontals. More than 100 fracture treatments were completed, with a very small percentage of screenouts. Approximately 30% of the wells penetrated “sweet spots” where the upper Vaca Muerta appears to be naturally fractured, and these wells produce without stimulation. After these high-pressure/high-productivity zones are penetrated, drilling operations can hardly continue because the wells become very difficult to control. At that point, the bottom-hole assembly is retrieved and the well is completed open hole.
The data-gathering strategy was to capture the right information at the right time. The collection plan involved acquiring the following static and dynamic data.
With this information at hand and with the goal of integrating all the available information, several reservoir-engineering techniques were applied to estimate the expected range of ultimate recovery per well.
Abstract Diagnostic fracture injection testing (DFIT) is an extremely popular technique to generate direct estimates of some of the key characteristics of shale reservoirs. DFIT analysis is generally done using analytical models traditionally developed for high-permeability reservoirs. The use of DFIT to analyze tight shale reservoirs introduces additional issues regarding applicability of these techniques for tight systems as well as operational issues such as long well shut-in times to obtain reasonable reservoir parameters. In this work we conduct geomechanics coupled reservoir flow simulations in order to forward model and analyze DFITs for tight shale gas reservoirs. This capability greatly enhances our ability to efficiently design the DFITs for tight shale reservoirs. In this work we simulate a typical DFIT using a geomechanics coupled reservoir flow simulator and generate the pre-closure and after-closure pressure response from the flow simulation model. This response is then analyzed using a standard Nolte pre-closure and after-closure analysis technique with an objective to evaluate the reservoir properties. We not only we show the validity of the Nolte analysis technique for tight rocks but we also provide guidelines on the amount of shut-in time required to generate a reasonable estimate of the reservoir properties from DFIT pressure response. We also show that the geomechanics coupled flow simulation of DFIT can provide estimates on fracture dimensions which compare reasonably with those given by more traditional fracture design tools. We demonstrate that the geomechanics coupled reservoir flow simulation provides an additional advantage over traditional fracture design tools in that it can numerically model the system response even after fracture closure.
Abstract The presence of fractures and faults play a significant role in recovery and performance of tight reservoirs exploited with hydraulically fractured wells. Faulting may result in asymmetric reservoirs, i.e. different quality reservoirs across the fault plane, due to the displacement of reservoir blocks along the fault plane. Typically, numerical well-test packages are used to match the pressure responses of such complex geology and well geometry. The limitations of such approaches in terms of ease of use and wide range of possible solutions plead for more attractive approach. Hence, here a semi-analytical approach has been followed to develop a new practically efficient flow solution for a well intersecting a finite conductivity vertical fracture in an asymmetric reservoir. The solution is characterised mainly by the bilinear flow resulting from formation and fracture linear flows. The pressure derivative curve exhibits a distinctive feature of an early fracture linear flow regime at a very early time reflecting the first fluid flow into the well from the fracture only. The shape of the derivative plot also suggests the characteristics of a bilinear flow, quarter slope, uttering the fracture characteristics, followed by a radial flow, zero slope, articulating the quality of the two reservoirs. Type curves of dimensionless time and pressure are presented along with field cases for vertical wells intersecting natural fractures or exploited by hydraulically fractures. The results of this paper enable reservoir engineers to carry out modelling of such complex reservoir/well scenarios with increasing certainty and long-term benefits and greater additional and favourable business impacts. Introduction Ramey (1976) and Raghavan (1977) have previously presented a review of the work done on flow along and toward fractures. They highlighted that intersecting fractures will strongly affect transient flow behavior of the well. Houze et al. (1984) described a well intersecting an infinite conductivity fracture in a naturally fractured reservoir simulated using a double-porosity model. Cinco-Ley and Samaniego (1978) presented a semi-analytical solution for the analysis of the transient pressure data of analysis for fractured wells in symmetric reservoirs, which is most likely to occur in the case of small fractures or strike-slip faults. Yet, in the case of reverse or normal faulting with large throw (Juxtaposing), different quality reservoirs could adjoin the fault plan. That is, faulting may result in a sudden displacement of rock along the fault plane that possibly yields, a large-scale slippage resulting in different quality fault blocks on both sides of the fault. Many production logs have shown two different fault-blocks resulting from a reverse fault that offset two zones sequence. Figure 1 illustrates a good example of faulting that juxtaposes different geology across the fault plane, whereby; two different quality zones are aligned through the fault plane. Here a semi-analytical solution for such a scenario is presented.