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To estimate the production potential at a new, prospective field site by means of simulation or material balance, one needs to collect various forms of costly field data and make assumptions about the nature of the formation at that site. Decline-curve analysis (DCA) would not be applicable in this scenario, because producing wells need to pre-exist in the target field. The objective of our work was to make first-order forecasts of production rates at prospective, undrilled sites using only production data from existing wells in the entire play. This is accomplished through the co-Kriging of decline-curve parameter values, where the parameter values are obtained at each existing well by fitting an appropriate decline model to the production history. Co-Kriging gives the best linear unbiased prediction of parameter values at undrilled locations, and also estimates uncertainty in those predictions. Thus, we obtained production forecasts at P10, P50, and P90, and we calculated the estimated ultimate recovery (EUR) at those same levels across the spatial domain of the play.
To demonstrate the proposed methodology, we used monthly gas-flow rates and well locations from the Marcellus shale-gas play in this research. Monitoring only horizontal and directional wells, the gas-production rates at each well were carefully filtered and screened. Also, we normalized the rates by perforation-interval length. We only kept production histories of 24 months or longer to ensure good decline-curve fits. Ultimately, we were left with 5,637 production records. Here, we chose Duong’s decline model (Duong 2011) to represent the production decline in this shale-gas play, and fitting this decline curve was accomplished through ordinary least-squares (OLS) regression.
Interpolation was done by universal co-Kriging while considering the correlation between the four parameters in Duong’s model, which also showed linear trends (the parameters showed dependency on the x and y spatial coordinates). Kriging gave us the optimal decline-curve coefficients at new locations (P50 curve), as well as the variance in these coefficient estimates (used to establish P10 and P90 curves). We were also able to map EUR for 25 years across the study area. Finally, the universal co-Kriging model was cross validated with a leave-one-out scheme, which showed significant, but not unreasonable, error in the decline-curve-coefficient prediction. The methods proposed were implemented and did not require various costly data, such as permeability and bottomhole pressure, thus giving operators a risk-based analysis of prospective sites. While we demonstrated the procedure on the Marcellus shale-gas play, it is applicable to any play with existing producing wells. We also made this analysis available to the public in a user-friendly web application (Xi and Morgan 2018).
Thermal shock occurs when a material's temperature is changed over a short period of time such that constituent parts of the material deform by different amounts. The deformation of material due to thermal load can be manifested through strain and stress. As the temperature diffuses from hydraulic fracture into reservoir, the temperature changes with x coordinate and the stress/strain can be obtained from the Equation (6). Once the stress at any point exceeds the strength of material, the body fails in one of the three modes of tension, compression or shear. A thermal load on rock, results in the creation and extension of cracks, crushing the grains, or sliding the grain interfaces. In this paper we look into the possibility of stimulating the rock matrix beyond hydraulic fracturing stimulation by cooling down the rock. The physics of temperature reduction in a solid dictates that when a solid is laterally fixed and undergoes temperature reduction, a thermal stress gradient is induced in the solid body. In rock, this thermal stress gradient leads to a differential contraction of the rock, which in turn creates openings, referred to as thermal cracks. We numerically solve the nonlinear gas diffusivity equation, using finite element method and show that the thermal cracks in rock have the potential to improve the productivity of wells placed in tight formations by 20%.
An analysis and evaluation of the Devonian Shale oil production's rapid decline rates and future potential production's rapid decline rates and future potential were performed. Various analysis techniques were employed to evaluate the performance of oil wells completed in the Lower Huron Member of the Devonian Shale in a seven-county area of western West Virginia and southeast Ohio. The analyses included material balance, and analysis of production data by a transient method and numerical simulation, together with statistical methods. Results of the evaluation showed that the coexistence of natural fractures and matrix permeability and porosity are necessary for oil production permeability and porosity are necessary for oil production from the Lower Huron in this area. Matrix permeability and porosity created by the inclusion of silt permeability and porosity created by the inclusion of silt in the shales are necessary for oil accumulation and production. Because the matrix permeability is production. Because the matrix permeability is extremely low, fractures are necessary to transmit the matrix production to the well bore. Although the cumulative oil volume is quite low, the recovery efficiency within the well drainage volume is significantly higher than expected for solution gas drive. A proposed explanation is that a mechanism of gas trapping is responsible for the high oil recovery efficiency. Another major finding of the study was that the methods of completion and stimulation had no effect on productivity. No major production problems except possible fracture closures were determined.
The study also showed that the combination of the geologic features necessary for oil production from shale is unique to this area but is by no means consistent over the entire area. In fact, the probability of locating the geological conditions necessary for economic oil production is only about 4 percent with current methods. Although unlikely, even if the tech nology for finding and producing oil from the Devonian Shale is greatly improved, few opportunities for commercial wells remain since the area is already densely drilled. Also, restimulation of old wells does not appear promising because wells are usually too close to abandonment pressure for additional oil production.