頁岩氣(shale gas)為開採潛能大的非傳統天然氣資源。在頁岩層中氣體除了以自由氣(free gas)形式儲存在頁岩的自然裂隙與頁岩基質孔隙中，也有部分氣體以吸附氣(adsorption gas)形式吸附於有機質物表面上。由於頁岩滲透率極低，一般需結合水平井與液裂處理技術(hydraulic fracturing)進行生產。本研究的目的是利用數值法研究頁岩氣層的生產特徵，包括研究不同地層特性(單孔隙及雙孔隙地層)、氣體流動機制(吸脫附及擴散機制)、以及完井方式(垂直井、水平井、及液裂處理)對井底流壓及產率變化行為之影響，以及研究不同時間下氣體的流動型態(flow regime)。並且假設不同的棄井壓力或經濟產率限制條件，研究頁岩氣層的估計最終採收量(estimated ultimate recovery, EUR)以及生產年限。
本研究首先建立單孔隙頁岩氣層數值模式，研究不同完井方式對井底流壓及生產率變化之影響；然後建立雙孔隙頁岩氣層數值模式，研究不同完井方式以及吸脫附與擴散機制對井底流壓及生產率之影響。在氣體流動型態方面，利用壓力微分以及產率倒數微分對時間變化圖，研究地層中氣體於不同時間下的流動行為。
研究結果包括：在估計最終採收量方面，(1) 單孔隙地層的垂直井、水平井、液裂井(垂直井考慮液裂處理)以及單液裂井(水平井考慮一組橫向液裂處理)的EUR低，僅三液裂井(水平井考慮三組橫向液裂處理)較高。在定產率(800MScf/day)操作生產下，當棄井壓力為1500以及1000psia時，三液裂井的EUR分別為1.9以及2.6Bcf。在定壓力( psi)操作生產下，當棄井產率為100以及50MScf/day時，三液裂井的EUR為1.2以及1.8Bcf。(2) 在雙孔隙地層中以不考慮吸脫附機制為例，以定產率生產當棄井壓力為1500以及1000psia時，EUR介於10.4~20.7Bcf以及13.9~26Bcf之間，以垂直井最低，依序為水平井、液裂井、單液裂井以及三液裂井。以定壓力生產當棄井產率為100以及50MScf/day時，EUR介於5.8~6.8Bcf以及6.3~6.8Bcf之間。(3) 雙孔隙地層考慮吸脫附機制的結果與不考慮吸脫附的結果相似。以定產率生產當棄井壓力為1500以及1000psia時，EUR介於11.4 ~23.6Bcf以及15.3~30.6Bcf之間。以定壓力生產當棄井產率為100以及50MScf/day時，EUR介於6.3~7.4Bcf以及6.9~7.4Bcf之間。(4) 液裂裂縫為氣體主要流通通道，當固定液裂裂縫大小與滲透率時，單液裂井的EUR與液裂井的結果相近；考慮不同裂縫數量時，三液裂井的EUR較單液裂井高。其中，在滲透率極低的單孔隙地層中若未進行液裂處理，則無法直接以垂直井或水平井進行生產。
在生產特徵方面：(1) 液裂井與單液裂井的井底流壓以及生產率隨時間變化之結果相近，且單液裂井的氣體流動型態與液裂井相同。三液裂井生產過程中，相鄰兩裂縫間的壓力傳遞互相干擾明顯，得到壓力微分圖之斜率值為0.64，介於地層線性流與偽穩態流之間。(2) 在雙孔隙地層中，不同完井方式的過渡帶期間可利用壓力微分以及產率倒數微分圖決定，結果顯示液裂處理對氣體進入與結束過渡帶的時間沒有影響。(3) 以Barnett頁岩為例，氣體吸附與擴散機制在頁岩氣層生產過程中影響不大，可直接以雙孔隙模式模擬頁岩氣層的生產行為。

Shale gas has the most development potential of unconventional gas resources. In shale gas reservoirs, the gas is stored both as free gas in the pore volume of natural fractures and the rock matrix, and as adsorbed gas on the surface of organic matter. Because of the ultra-low permeability of shale, hydraulic fracturing and horizontal wells are used for production. The purpose of this study is to use the numerical simulation method to study the effect of flowing bottomhole pressure (BHP), production rate, and the flow regimes of shale gas reservoirs on different reservoir types (single- and dual- porosity systems), gas-flow mechanisms (adsorption and diffusion), and well completion methods (vertical well, horizontal well, and hydraulic fracturing).The estimated ultimate recovery (EUR) and production years were examined by assuming different abandonment pressures or rates.
A single-porosity model was first established to study the effect of BHP and production rate on different well completion methods, and then a dual-porosity model, to study the effect of BHP and production rate on different well completion methods and the adsorption and diffusion mechanisms. To study gas-flow regimes, both the pressure-derivative and reciprocal-rate derivative methods were used.
The following results (EUR and production characteristics) were obtained from a reservoir with average properties from literature. Estimated Ultimate Recovery (EUR) results were: (1) for single-porosity system, the EURs of the vertical, horizontal, fractured vertical, and the single-fractured horizontal wells were very low except the three-fractured horizontal well. For constant-rate production (800MScf/day), the three-fractured horizontal well was 1.9 and 2.6Bcf at the abandonment pressures of 1500and 1000psi, respectively. For constant pressure ( psi) production, the three-fractured horizontal well was 1.2 and 1.8Bcf at the abandonment rates of 100 and 50MScf/day. (2) For dual-porosity constant-rate production, EUR ranges were of 10.4~20.7Bcf and 13.9~26Bcf at the abandonment pressures of 1500 and 1000psi. The three-fractured horizontal well had the highest value, then the single-fractured horizontal, fractured vertical, horizontal, and vertical wells. For constant-pressure production, EUR ranges were 5.8~6.8Bcf and 6.3~6.8Bcf at the abandonment rates of 100 and 50MScf/day. (3) For dual-porosity considering gas adsorption and diffusion constant-rate production, EUR ranges were 11.4~23.6Bcf and 15.3~30.6Bcf at the abandonment pressures of 1500 and 1000psi. For constant-pressure production, EUR ranges were 6.3~7.4Bcf and 6.9~7.4Bcf at the abandonment rates of 100 and 50MScf/day. (4) Gas could not be produced using the ultra-low permeability single-porosity system without hydraulic fracturing. The EUR of the vertical-fractured well was similar to that of the single-fractured horizontal well, but the fracture size and permeability were the same. Three-fractured horizontal well had higher EUR than did single-fractured horizontal well.
Production characteristics were: (1) BHP, production-rate behavior, and flow regimes of vertical-fractured and single-fractured horizontal wells were almost the same. The slope of the pressure-derivative plot was 0.64 because of the pressure interference between adjacent fractures during production from three-fractured horizontal well. (2) Transition time in dual-porosity systems of different well completions can be determined from both the pressure- and the reciprocal-rate derivative plots, which shows that hydraulic fracturing has no effect on transition periods. (3) Gas adsorption and diffusion mechanisms had little effect on Barnett shale during production; therefore, shale gas production behavior can be directly modeled using the dual-porosity system.

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