Development and Application of Hydraulic Fracture Propagation Model Using Multiple Planar Fracture with Mixed Mode in Naturally Fractured Shale Reservoir

Abstract

Hydraulic fracturing work in naturally fractured shale reservoir causes complex fracture geometry since hydraulic fracture complicatedly propagates corresponding to the existence of natural fracture as well as geomechanical factors. For hydraulic fracture propagation modeling, in the past, single planar fracture approach denotes fracture propagating only in the direction perpendicular to horizontal well regardless of the existence of natural fracture. Recently, the model implements multiple planar fracture being able to describe the propagation more realistically. In this model, as fracture crossing criterion between hydraulic fracture and natural fracture, the hydraulic fracture propagation is generally assumed to be a multiple planar fracture with opening mode. However, when hydraulic fracture encounters natural fracture nonorthogonally, it might not be directly crossed natural fracture. The tip of hydraulic fracture may slide slightly parallel to natural fracture face, and then propagate through rock mass according to the stress difference between hydraulic fracture tip and natural fracture face, therefore it is essential to include sliding mode. This study proposes a new multistage hydraulic fracture propagation model using multiple planar fracture with mixed mode by linearly superposing opening mode and sliding mode. This model is then coupled with commercial reservoir flow simulator through grid mapping process in the form of discrete fracture network developed in this work. The modeling results for the verification about hydraulic fracture crossing natural fracture excellently matches with experimental results for various cases of intersection angle and difference of maximum and minimum horizontal stresses. The model is run for the investigation of sensitivity analysis for the parameters of inclination angle, frictional coefficient of fracture interface, and natural fracture orientation. From the results, hydraulic fracture passes through natural fracture appropriately corresponding to crossing criterion, and thereafter, the fracture propagates appropriately with respect to fracture reinitiation angle. The model of this study is then compared to the model with opening mode and also the model of single planar fracture approach. The result shows that there is a large discrepancy in stimulated reservoir volume, because of a number of intersections between fractures (meaning fracture connectivity). In the application of the model for Barnett shale reservoir, the stimulated reservoir volume of the model of this study and commercial model are calculated differently which indicates that the model of this study is significant in evaluating the initial gas in place estimated by stimulated reservoir volume.| 자연균열 셰일저류층에서 수압파쇄공법 수행 시, 셰일층의 지력학적 조건뿐만 아니라 셰일층 내 존재하는 자연균열에 따라서 수압파쇄균열이 전파하기 때문에 복잡한 균열망 구조가 형성된다. 이러한 시스템에서의 파쇄균열 전파모델링 연구와 관련하여 과거에는 자연균열의 존재 여부와 관계없이 파쇄균열이 수평정에 수직 방향으로만 전파되는 단일평판형 균열개념이 수압파쇄모델에 적용되었다. 최근에는 보다 현실적인 균열전파 묘사를 위해 다중평판형 균열개념이 적용된 수압파쇄모델 연구가 진행되고 있다. 이 모델들은 파쇄균열 전파묘사에서 파쇄균열이 자연균열과 만났을 때 통과 여부 결정 기준에서 열림 모드만을 적용한 다중평판형 균열개념만을 가정한다. 그러나 파쇄균열이 자연균열과 비수직으로 만날 경우에는 자연균열을 바로 통과하지 못한다. 즉, 파쇄균열의 끝단이 자연균열면을 따라 미끄러진 후, 파쇄균열의 끝단과 자연균열면 사이에 작용하는 응력의 차이에 따라 파쇄균열이 암반으로 전파하게 된다. 따라서 이러한 경우에는 미끄러짐 모드도 포함하는 것이 필수적이다. 본 연구에서는 열림 모드와 미끄러짐 모드가 선형 중첩된 혼합 모드의 다중평판형 균열개념을 적용한 새로운 다단계 수압파쇄균열전파모델을 제안하였다. 본 모델을 통해 생성된 개별균열망을 그리드 맵핑 과정을 통해 상용 저류층 유동시뮬레이터와 연동이 되도록 하였다. 본 모델의 검증을 위해 다양한 교차각과 최대-최소수평응력 차이에 대한 균열통과 기준에 대해서 이전에 발표된 실험결과와 비교한 결과, 모든 조건에서 모델링 결과가 동일하게 산출되었다. 모델의 민감도 분석을 수행한 결과에서는 경사각, 균열면의 마찰계수, 자연균열의 방향성에 대한 분석 결과, 파쇄균열이 균열통과 기준에 따라 적정하게 자연균열을 통과하고, 그 이후 균열의 재발생 기준에 따라 파쇄균열이 전파됨을 확인하였다. 또한 본 연구에서 제안한 혼합 모드의 다중평판형 균열모델을 열림 모드만을 적용한 모델과 단일평판형 균열모델과 비교하였다. 그 결과, 세 가지 접근방법에 의하여 각각 산출되는 균열의 연결성과 관련된 균열의 교점수 때문에 생산정자극부피에서 차이가 있음을 확인하였다. 본 모델의 응용 측면에서 바넷 셰일저류층의 자료를 이용하여 본 모델과 상용 모델에서 생성된 균열시스템에 대한 유동시뮬레이션을 수행하였다. 그 결과 가스회수율과 생산정자극부피가 각각 다르게 산출되었으며, 이는 본 연구의 혼합 모드를 적용한 다중평판형 균열모델이 생산정자극부피에 의한 원시가스매장량 평가 시 매우 중요함을 알 수 있다.; Hydraulic fracturing work in naturally fractured shale reservoir causes complex fracture geometry since hydraulic fracture complicatedly propagates corresponding to the existence of natural fracture as well as geomechanical factors. For hydraulic fracture propagation modeling, in the past, single planar fracture approach denotes fracture propagating only in the direction perpendicular to horizontal well regardless of the existence of natural fracture. Recently, the model implements multiple planar fracture being able to describe the propagation more realistically. In this model, as fracture crossing criterion between hydraulic fracture and natural fracture, the hydraulic fracture propagation is generally assumed to be a multiple planar fracture with opening mode. However, when hydraulic fracture encounters natural fracture nonorthogonally, it might not be directly crossed natural fracture. The tip of hydraulic fracture may slide slightly parallel to natural fracture face, and then propagate through rock mass according to the stress difference between hydraulic fracture tip and natural fracture face, therefore it is essential to include sliding mode. This study proposes a new multistage hydraulic fracture propagation model using multiple planar fracture with mixed mode by linearly superposing opening mode and sliding mode. This model is then coupled with commercial reservoir flow simulator through grid mapping process in the form of discrete fracture network developed in this work. The modeling results for the verification about hydraulic fracture crossing natural fracture excellently matches with experimental results for various cases of intersection angle and difference of maximum and minimum horizontal stresses. The model is run for the investigation of sensitivity analysis for the parameters of inclination angle, frictional coefficient of fracture interface, and natural fracture orientation. From the results, hydraulic fracture passes through natural fracture appropriately corresponding to crossing criterion, and thereafter, the fracture propagates appropriately with respect to fracture reinitiation angle. The model of this study is then compared to the model with opening mode and also the model of single planar fracture approach. The result shows that there is a large discrepancy in stimulated reservoir volume, because of a number of intersections between fractures (meaning fracture connectivity). In the application of the model for Barnett shale reservoir, the stimulated reservoir volume of the model of this study and commercial model are calculated differently which indicates that the model of this study is significant in evaluating the initial gas in place estimated by stimulated reservoir volume.