Experimental Study of Hydraulic Fracture Propagation Behavior during Multistage Fracturing in a Directional Well

Due to the limited space of offshore platform, it is unable to implement large-scale multistage hydraulic fracturing for the horizontal well in Lufeng offshore oilfield. Thus, multistage hydraulic fracturing technology in directional well was researched essentially to solve this problem. Modeling of fracture propagation during multistage fracturing in the directional and horizontal wells in artificial cores was carried out based on a true triaxial hydraulic fracturing simulation experiment system. The effects of horizontal stress difference, stage spacing, perforation depth, and well deviation angle on multifracture propagation were investigated in detail. Through the comparative analysis of the characteristics of postfrac rock and pressure curves, the following conclusions were obtained: (1) multistage fracturing in horizontal wells is conducive to create multiple transverse fractures. Under relatively high horizontal stress difference coefficient (1.0) and small stage spacing conditions, fractures tend to deflect and merge due to the strong stress interference among multiple stages. As a consequence, the initiation pressure for the subsequent stages increases by more than 8%, whereas in large stage spacing conditions, the interference is relatively lower, resulting in the relatively straight fractures. (2) Deepening perforation holes can reduce the initiation pressure and reduce the stress interference among stages. (3) When the projection trace of directional wellbore on horizontal plane is consistent with the direction of the minimum horizontal principal stress, fractures intersecting the wellbore obliquely are easily formed by multistage fracturing. With the decrease of well deviation angle, the angle between fracture surface and wellbore axis decreases, which is not conducive to the uniform distribution of multiple fractures. (4) When there is a certain angle between the projection trace of directional wellbore on horizontal plane and the direction of minimum horizontal principal stress, the growth of multiple fractures is extremely ununiform and the fracture paths are obviously tortuous.

Numerical Analysis of Multiple Factors Affecting Hydraulic Fracturing in Heterogeneous Reservoirs Using a Coupled Hydraulic-Mechanical-Damage Model

Hydraulic fracturing performance, affected by multiple factors, was essential to the economic exploitation of oil and gas in heterogeneous unconventional reservoirs. Multifactor analysis can gain insight into the fracturing response of reservoirs and in turn optimize the treatment design. Based on characterizations of the geological setting of a heterogeneous glutenite reservoir, the hydraulic fracture (HF) initiation and propagation process, as well as the stimulated reservoir volume (SRV), were simulated and analyzed using a coupled hydraulic-mechanical-damage model. The Weibull distribution was employed to describe rock heterogeneity. The numerical model was verified with microseism (MS) interpretation results of HF geometry. A multifactor analysis and optimization workflow integrating response surface methodology, central composite design (CCD), and numerical simulations was proposed to investigate the coupling effects of multiple geomechanical and hydrofracturing factors on SRV and identify the optimum design of fracturing treatment. The results showed that the horizontal stress difference and injection rate were the most significant factors to control the SRV. Increasing the injection rate and reducing fluid viscosity may contribute to improving the SRV. It is more difficult to increase the SRV at higher horizontal stress difference than at lower horizontal stress difference. The multifactor analysis and optimization workflow introduced in this work was a practical and effective method to control the HF geometry and improve the SRV. This study provided a deep understanding of the hydraulic fracturing mechanism and possessed theoretical significance for treatment design.

Study on Fracture Initiation Mechanisms of Hydraulic Refracturing Guided by Directional Boreholes

In order to generate a new fracture far away from the original fracture in refracturing and effectively enhancing productivity, the technology of hydraulic refracturing guided by directional boreholes was presented. The effects of induced stress generated by the original hydraulic fracture, fracturing fluid percolation effect, wellbore internal pressure, and in situ stress on stress field distribution around wellbore were considered to obtain a fracture initiation model of hydraulic refracturing guided by two directional boreholes. The variation of maximum principal stress (σmax) under different conditions was investigated. The researches show that the directional boreholes result in a “sudden change region” of maximum principal stress around wellbore, reflecting dual stresses effects from vertical wellbore and directional boreholes on the rock. The width of sudden change region decreases as the distance from wellbore increases. Due to sudden change region, the refracturing fracture tends to initiate around directional boreholes. Whether the new fracture initiates and propagates along directional boreholes depends on comprehensive effect of borehole azimuth, borehole diameter, borehole spacing, horizontal stress difference, height, and net pressure of original fracture. The specific initiation position can be calculated using the theoretical model proposed in this paper. Affected by induced stress of the original fracturing, the rock tends to be compressed during refracturing, i.e., increased fracturing pressure. Sensitivity analysis with “extended Fourier amplitude sensitivity test (EFAST)” method shows the initiation of new fracture is mainly controlled by directional boreholes parameters and has little relation with in situ stress and parameters of original fracture. The influence rank of each parameter is as follows: borehole diameter > borehole spacing > original fracture net stress > borehole azimuth > horizontal stress difference > original fracture height. During design of refracturing, in order to better play the role of directional boreholes, and create a new fracture far away from original fracture, the optimal design is conducted with measures of optimizing boreholes azimuth, increasing borehole diameter and reducing borehole spacing if conditions permit. The research provides the theoretical basis for hydraulic refracturing guided by directional boreholes, which is helpful for the design of fracturing construction programs.