A Unified Approach To Optimize Fracture Design of a Horizontal Well Intercepted by Primary- and Secondary-Fracture Networks

Abstract

Summary The classical optimization design dependent on a single-fracture (SF) assumption is widely applied in performance optimization for hydraulically fractured wells. The objective of this paper is to extend the optimal design to a complex fracture network to achieve the maximum productivity index (PI). In this work, we established a pseudosteady-state (PSS) productivity model of a fractured horizontal well, which has the flexibility of accounting for the complexity of fracture-network dimensions. A semianalytical solution was then presented in the generalized matrix format through coupling reservoir- and fracture-flowing systems. Subsequently, several published studies on the PSS productivity calculation of a SF were used to verify this model, and a 3D transient numerical simulation of an orthogonal fracture network was used to perform further verification. We show that results from our solutions agree very well with those benchmarked results. On the basis of the model, we provide a detailed analysis on the productivity enhancement of the fracture-network/optimization work flow using unified fracture design (UFD). The results show the following: The PI is determined by fracture conductivity and complexity (network size, spacing, and configuration), and it is a function of fracture complexity and conductivity when the influence of proppant volume is not considered. Under the constraint of a given amount of proppant known as UFD, the maximum PI would be achieved when the best balance between network complexity and conductivity was obtained. It is more advantageous to minimize fracture complexity by creating relatively simple-geometry fractures with smaller network size and larger fracture spacing in the condition of small and intermediate proppant numbers. It should be the design goal to generate a complex network by creating relatively complex-geometry fractures with larger network size and smaller fracture spacing in the condition of a large proppant number. Increasing fracture complexity could reduce the optimal requirement of fracture conductivity. The proposed approach can provide guidance for a network-hydraulic-fracturing design for an optimal completion.

Characterization and Modelling of Multiscale Natural Fractures in Shale Reservoirs: A Case Study from a Block in the Southern Sichuan Basin

“…Natural fractures are the basis for the formation of complex fracture networks [1][2][3][4][5]. Fracture characterization and modelling of shale reservoir is the key to the rational design of shale gas horizontal well drilling and completion [6][7][8][9] and accurate evaluation of reservoir productivity [10][11][12][13][14].…”

Productivity analysis for a horizontal well with multiple reorientation fractures in an anisotropic reservoir

“…Sorek et al [20] extended the UFD method to maximize the productivity index of the horizontal well with multiple acute-angle transverse fractures and concluded that fracture width and fracture length depend on fracture angle and proppant number. Wang et al [21] applied the UFD method to calculate the productivity index of a horizontal well with fracture networks in a closed rectangular reservoir and stated that the productivity index reaches the peak when fracture networks penetrate throughout the seepage area under infinite conductivity. Asadi et al [22] combined the UFD method with Direct Boundary-Element Method (DBEM) to evaluate the influence of fracture-fluidleak off on the productivity index of multiple-fracture horizontal wells and found that the optimal fracture with leak off should be shorter and wider for a fixed proppant number than the case without leak off.…”

“…The previous models were established on the assumption of the ideal planar fractures [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In fact, many complex fractures can be formed in tight reservoirs, such as multi-wing fractures, non-planar fractures and reorientation fractures [25][26][27][28].…”

Productivity-Index Behavior for a Horizontal Well Intercepted by Multiple Finite-Conductivity Fractures Considering Nonlinear Flow Mechanisms under Steady-State Condition

“…For a multi-fractured horizontal well, the sequence of flow-regimes may be more complex (Figure 1) [4]. At some point after compound linear flow caused by the fracture interference, a compound elliptical flow regime (regime 5 in Figure 1a) would appear as the transient pressure waves move past the ends of the fractures [5,6]. Put another way, the compound elliptical response is the pseudo-steady state in an infinite drainage area.…”