煤層氣地層是指含有天然氣的煤層，因為地層構造為雙孔隙地層，且生產（或流動）機制比天然氣層多了吸附機制及擴散機制，所以分析上較複雜。在煤層氣生產過程中可測得的壓力資料包括暫態壓力及平均壓力，但不論暫態壓力或平均壓力，其壓力變化皆與煤層氣的生產機制（吸附及擴散機制）有關。所以，本研究目的為分析生產機制對暫態壓力及平均壓力之影響，觀察不同參數改變吸附及擴散能力對暫態壓力的影響以及對利用平均壓力由物質平衡法推估原始埋藏量的差異。
　　在生產機制對暫態壓力之研究上，首先分析單孔隙及各種雙孔隙傳輸的暫態壓力變化，並利用所建立之雙孔隙菲克擴散數值模式改變影響生產機制之參數，進行暫態壓力影響研究，最後利用傳統雙孔隙達西流動解析法分析雙孔隙菲克擴散之數值模式，反推導求得影響過渡期壓力變化的參數（儲存容積比及竄流係數）並進行分析。
　　在生產機制對平均壓力之研究上，本研究利用含有瞬間擴散的數值模式所得之結果（平均壓力與累積產量關係），利用修正物質平衡法估算原始埋藏量，結果可與體積法及數值模式的計算結果互相比對驗證正確性。最後，進行生產機制對傳統與修正物質平衡法計算結果之影響研究。
　　根據單孔隙及各種雙孔隙傳輸的暫態壓力變化得知，煤氣層中的壓力變化所出現的雙孔隙達西流動行為，主要是因為煤層氣地層的生產機制所造成。另外，利用傳統雙孔隙達西流動解析法分析可知，若改變煤基氣體埋藏量的參數，會同時改變不同孔隙系統的煤層氣儲量及煤基濃度變化而改變儲存容積比與竄流係數；若改變不同孔隙系統傳輸能力的參數，只會改變竄流係數。不論有沒有考慮擴散機制，利用平均壓力由傳統物質平衡法估算原始埋藏量，會得到遠低於正確的原始埋藏量。經過修正後得到適用於瞬間擴散煤層氣的修正物質平衡式，不考慮擴散機制下，根據平均壓力利用修正物質平衡法估算原始埋藏量可得正確之結果，考慮擴散機制後，擴散時間常數越大，p/z*隨Gp變化在流動穩定後會呈直線且向下偏移，且估算出的原始吸附氣量越低，但差異不大，可知擴散機制對修正物質平衡法影響不大。

The production mechanisms of coalbed methane include adsorption and diffusion, which do not exist in conventional natural gas reservoir. Both transient pressure and average pressure data can be obtained from well testing and production testing in coalbed methane reservoir. Transient pressures data can be used for evaluation of reservoir parameters. The average pressure data are often used in material balance method for the estimation of original gas in place. Both transient pressure and average pressure are affected by the production mechanisms (adsorption and diffusion) of the coalbed methane reservoir. Therefore, the purpose of this study is to analyze the effect of production mechanisms to the transient pressure and average pressure. The sensitivity of adsorption and diffusion to the transient pressure has been conducted. And the effect of the production mechanism to the original gas in place by material balance is also studied.
Numerical models are set up for this study, including conventional gas reservoir, dual porosity and dual porosity with diffusion and adsorption. These models are used to generate pressure transient data, which are analyzed by pressure derivative type curves, for studying pressure behavior and reservoir parameters of coalbed methane reservoir. Also, the numerical model is been used to study how the dual porosity with Fick diffusion flow effects the transient pressure behavior. And the analytical solution of traditional dual porosity Darcy flow is also used, to analyze the transient pressures from the numerical model of dual porosity with Fick diffusion flow, and obtain storage ratio and interporosity flow parameter.
In the study of the effect of production mechanisms to the average pressure, the results of numerical model, with instant diffusion, are used to estimate the original gas in place by modified material balance, which is suitable for instant diffusion. The estimated original gas in place from modified material balance method, are validated with those from volumetric method and numerical simulation. The results from modified material balance are also compared with those from traditional material balance method.
The results from the studies of transient pressure behavior of single-porosity and multiple-porosity, the pressure behavior of CBM reservoir is dual porosity Darcy flow. The pressure behavior is related to the production mechanisms of CBM reservoir. The study also shows that the storage ratio and interporosity flow parameter will be changed, if any parameters are related to the original gas in place, because the matrix concentration is changed.
Whether or not the diffusion mechanism has been taken into consideration, the original gas in place estimated by material balance from average pressure will be much lower than the correct original gas in place. For the case of no diffusion mechanism, the estimation of the original gas in place by modified material balance with average pressure is correct. However, for the case of diffusion mechanism with long sorption time, the data points with p/z* versus Gp plot will be shifting downward. Thus, the estimated original gas in place will be under estimated.

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