This paper identifies the need for the development of specialized solutions for pressure transient analysis in coal seams and, subsequently, provides a comprehensive solution package for these reservoirs. The proposed solutions are applicable to radial systems with finite wellbore radius and with either an infinite outer boundary, a constant pressure outer boundary or a closed outer boundary. At the inner boundary constant pressure or constant terminal rate cases can be specified.The single-phase flow formulation for coalbed methane includes non-equilibrium sorption phenomena in the coal matrix and laminar gas transport in the natural fracture network (cleat system). The transport of methane in the coal matrix is described by using spherical elements. The mathematical models which account for unsteady state and pseudosteady state matrix/fissure methane transfer are incorporated to the transport equation.In order to obtain a closed form solution in real time domain and avoid the pitfalls of numerical inversion approximate inversions of the solutions from the Laplacian space are also developed. These closed form solutions are found to be in good agreement with the numerically inverted solutions. The effects of different models of matrix/fissure flow on the solution are also discussed. The validity of the proposed analytical solutions are checked against a numerical simulator and comparisons showing good agreements between the two are presented.The solution package describes forward solutions which have the potential to be used in an interpretation method for insitu characterization of coal seams and other gas reservoirs where adsorption and desorption phenomena are considered to be active.

A solution package that describes the pressure transient behavior of coal reservoirs in the presence of a hydraulically fractured well is presented. In the development of the solutions uniform-flux vertical fracture model is employed. The proposed solutions are then extended to describe the pressure transient behavior of composite coalbed reservoirs with a vertical well which may have a vertical fracture. The proposed composite system solution has potential applications when "tailored-pulse" fractures or damage exist around the degasification well. In this study, dual porosity nature of coal seams is modeled by describing micropore structure by spherical matrix elements. The formulation employed includes single-phase laminar gas transport in the natural fracture network (cleat system). Flow within the micropores is described by Fick's law of diffusion. Matrix to fissure flow mechanism is modeled by unsteady-state sorption/diffusion formulation. The equilibrium isotherm which defines the amount of desorbed gas is obtained through Langmuir's theory. The solutions are obtained analytically in the Laplacian space and inverted to the real time domain via Stehfest algorithm. The proposed solutions are tested against the subsets of the problem with known solutions and in each case excellent agreements are observed. Furthermore, the pressure transient data generated by a finite difference coal seam degasification model are used to check the validity of the solutions presented. Sensitivity of the solutions to a wide range of coal seam properties are also included. The solutions presented in this paper equip the well test analysis engineer with a precise tool to analyze the pressure transient behavior of the hydraulically fractured degasification wells. The solution package encompasses infinite. constant pressure and no-flow outer boundary conditions together with constant flow rate specification at the wellbore. Introduction Pressure transient behavior of coal seams differ significantly from conventional gas reservoirs. The existence of the adsorption phenomena together with the dual porosity nature of the coalbed reservoirs call for the development of solutions which are applicable to pressure transient analysis of the coal seam degasification wells. In addition to the gas stored in the pore volumes, coal seams store gas as an adsorbed mono-molecular layer on coal grain surfaces. The amount of gas in adsorbed stage is defined by the equilibrium isotherm. Because of small pore diameter in the coal matrix the flow of desorbed gas from coal grain surface to the cleat system is governed by Fick's law of diffusion. Once gas reaches to the cleat system flow is laminar and governed by Darcy's law. Hydraulic fracturing is an effective well stimulation technique employed in production of oil and gas from damaged wells or wells producing from low permeability reservoirs. The vicinity of a vertical well drilled in a coal seam may be damaged by invasion of the natural fracture network with drilling fluid and cement. The presence of cement over 100 ft from the wellbore has been noted in mine-through operations in coal seams. Hydraulic Fracturing of vertical wells drilled in coal seams is employed to bypass wellbore damage. A hydraulically induced fracture will also be effective in connecting the fracture network to the wellbore, accelerating and extending the pressure drawdown into the reservoir, and reducing production of fines by reducing the pressure drop in the near-wellbore area where the coal is often in a degraded condition. P. 407^

In early stages of reservoir depletion, it is often a challenging task to accurately determine reservoir properties that are representative of the actual field. Due to different scales of data obtained from various sources like seismic data, well logs, cores, and production data, there is a lot of uncertainty in solving the inverse problem of estimating formation rock and fluid properties from the field data. Hard-computing protocols like reservoir simulation are time and labor intensive. The objective of the current study is to develop a reservoir characterization tool using a novel approach of correlating seismic attributes with well logs and production data using artificial intelligence approach. The tool will enable construction of spatial oil maps at different times revealing sweet spots and aid in optimized field development planning. A workflow is developed for devising a comprehensive reservoir characterization tool based on artificial expert systems. A case study of an offshore deep-water asset is used in demonstrating the tenets of the workflow. The reservoir under consideration is highly heterogeneous in terms of property distribution and is believed to be highly channelized. The ANN based tool assists in identifying sweet spots by predicting optimal well location/completion parameters and production profiles. The multilayer feedforward back-propagation based neural network tool developed is able to capture the correlations that exist amongst seismic data, well logs, completion data, and production data. Well logs are correlated to seismic attributes and geometric location of wells with an average testing (blind test) error of less than 20%. Having correlated seismic data with well logs, synthetic well logs are generated for the entire area of seismic coverage. Synthetic well logs combined with seismic data are able to correlate well with the production within 21% error. The tool developed enables users to predict entire well log suites for even a directional well of user defined configuration through a graphic user interface in a short period of time (typically less than a minute). This methodology uses a unique way of computing seismic attributes following a horizontal well path and correlating them with the suite of well logs. Incorporation of interference effect from neighboring producers and injectors, schedule of production and functional links based on geographic location has made the production performance module robust and reliable. The workflow enables generation of oil production forecast maps through production performance network. NPV (net present value) calculations integrated with production forecasts is used in identifying the potential infill well locations. The results discussed in the paper showcase the robust nature of the methodology.

A solution package that describes the pressure transient behavior of coal reservoirs in the presence of a hydraulically fractured well is presented. In the development of the solutions uniform-flux vertical fracture model is employed. The proposed solutions are then extended to describe the pressure transient behavior of composite coalbed reservoirs with a vertical well which may have a vertical fracture. R e proposed composite system solution has potential applications when "Tailored-pulse" fractures or damage exist around the &gasification well.In this study, dual porosity nature of coal seams is modeled by &scribing micropore structure by spherical matrix elements. The formulation employed includes single-phase laminar gas transport in the natural fracture network (cleat system). Flow within the micropores is described by Fick's law of diffusion. Matrix to fissure flow mechanism is madeled by unsteady-state sorption/diffusion formulation. The equilibrium isotherm which defines the amount of &sorbed gas is obtained through Langmuir's theory. The solutions are obtained analytically in the Laplacian space and inverted to the real time domain via Stehfest algorithm.The proposed solutions are tested against the subsets of the problem with known solutions and in each case excellent agreements are observed. Furthermore, the pressure transient data generated by a finite difference coal seam degasifidon model are used to check the validity of the solutions presented. Sensitivity of the solutions to a wi& range of coal seam properties are also included.The solutions presented in chis paper equip the well test analysis engineex with a precise tool to analyze the pressure transient behavior of the hydraulically fractured degasification wells.The solution package encompasses infinite, constant pressure and no-flow outer boundary conditions together with constant flow rate specification at the wellbore.

A simplified analysis technique to determine the desorption characteristics of coal seams using pressure transient analysis is presented. The proposed technique is based on formulation of presented. The proposed technique is based on formulation of single phase gas flow in coal seams with unsteady state sorption/diffusion phenomena in coal matrix and laminar flow in cleat system, The matrix geometry is represented by using spherical elements. A line source solution which defines the pressure distribution in an infinite-acting radial-cylindrical coal seam is used in the development of the proposed analysis technique. The simplified analysis presented in this paper is first tested against a previously developed closed form solution which describes the pressure transient behavior of coal seams. The proposed solution pressure transient behavior of coal seams. The proposed solution shows excellent agreement with this more rigorous solution, and its versatility is demonstrated through a simple analysis procedure which does not require the use of computationally procedure which does not require the use of computationally laborious functional groups. Furthermore, a pressure transient data generated by a coal seam degasification numerical model is analyzed successfully using the methodology presented here. As an example application of the proposed technique to the analysis of drawdown test data is also included. The proposed inverse solution procedure which is devised as a viable pressure transient analysis technique is applicable to coal seams and other unconventional gas reservoirs where adsorption/desorption phenomena are effective. The methodology introduced in this paper has the potential of providing the necessary tools that can be used in in-situ determination of the transport properties and the sorption characteristics of the coalbed methane reservoirs. Introduction Pressure transient analysis is a powerful tool for in-situ characterization of oil and gas reservoirs, and has similar potentials to be used in in-situ determination of transport and sorption characteristics of coal seams. However, existence of a natural fracture network and presence of time dependent sorption phenomena result in a relatively challenging problem of developing a pressure transient analysis technique specifically applicable to coalbed methane reservoirs. Because of the intricate nature of the flow of methane in coal seams the mathematical description of the phenomena is rather more demanding. Some researchers proposed the use of empirical models which are based on the simple mathematical descriptions of the physical phenomena observed. These empirical models are relatively practical but they lack the theoretical rigor required for accurate predictions. Single-porosity models applied to describe methane flow in coal seams employ partial differential equations valid for conventional reservoirs with some modifications such as inclusion of a pressure dependent source term or the modification of the accumulation term. These single-porosity models utilize equilibrium sorption models which do not account for the time dependence of the sorption/diffusion process in the micropore structure of the coal and this results in process in the micropore structure of the coal and this results in predictions of higher flow rates an/or higher reservoir predictions of higher flow rates an/or higher reservoir pressures. pressures. The time dependence of transport of methane in the micropores is taken into consideration in non-equilibrium sorption/diffusion models which are essentially obtained by modifications implemented to the conventional dual-porosity formulations. In dual-porosity approach the mathematical formulation is constructed by a set of coupled equations representing the two-stage flow of methane in the coal seam. While the pseudosteady-state non-equilibrium models are similar to the Warren and Root model of conventional dual-porosity reservoirs, unsteady-state sorption models are obtained by adaptation of the conventional dual-porosity model of De Swaan. The long term predictions of these two non-equilibrium sorption models do not differ significantly, but as concluded by Spencer et al. the early-time predictions obtained using pseudo-steady-state approach may not be accurate. King and Ertekin stated that the more rigorous unsteady-state approach should be taken into consideration in pressure transient analysis applications. Unlike conventional gas reservoirs coal seams, in addition to the gas stored in the pore volumes, also store methane gas as an adsorbed mon-molecular layer on coal grain surfaces. The volume of gas in adsorbed state is controlled by the sorption isotherm. Two different sorption isotherms that are commonly used in formulating the amount of adsorbed gas are Henry's and Langmuir's isotherms. While Henry's law predicts a linear isotherm, Langmuir's theory constructs a non-linear sorption isotherm which supports the mono-molecular layer adsorption presumption. presumption. P. 43