Interpreting Multistage Minifrac Tests using Discrete Element Modeling of Foliated Rock with Various Drilling Mud Properties

Pál Farkas, Márton (GFZ - German Research Centre for Geosciences, Golder Associates Hungary) | Dankó, Gyula (Golder Associates Hungary) | Yoon, Jeong Seok (GFZ - German Research Centre for Geosciences) | Zang, Arno (GFZ - German Research Centre for Geosciences) | Zimmermann, Günter (GFZ - German Research Centre for Geosciences) | Stephansson, Ove (GFZ - German Research Centre for Geosciences)



Multistage mini hydraulic fracturing tests were performed in a borehole located in central Hungary in order to determine in-situ stress. At depth of about 500 to 560 meters, observed pressure versus time curve in metamorphic rock (mica schist) show a typical results. After each pressurization cycle, the fracture breakdown pressure in the first fracturing cycle is lower than the reopening pressures in the subsequent reopening and step-rate phases. It is assumed that the composition of the drilling mud and observed foliation of the mica schist have a significant influence on the pressure values. In order to investigate this problem, numerical modeling was performed using the discrete element code (ITASCA Particle Flow Code, PFC), which has been proven as an effective tool to investigate rock engineering problems associated with hydraulic fracturing. The code presented in this study enables simulating hydro-mechanically coupled fluid flow in crystalline rock with low porosity and pre-existing fractures (represented by the smooth joint contact model in PFC) in two dimensions. In this study, the sensitivity of the effect of foliation angle and fluid viscosity on the peak pressure is tested. The anomalous characteristics of the pressure behavior are interpreted in that way that the drilling mud penetrates the sub-horizontal foliation plane, it clogs the plane of weakness and makes the opened fracture tight. Eventually, the process prevents leak-off from the opened fracture that might explain the increased fracture reopening pressure in subsequent cycles.

1. Introduction

Hydraulic-driven fractures play a key role in energy technologies. In particular, unconventional oil, shale gas and geothermal reservoirs are often characterized by in-situ rock with intrinsic low permeability. Prior to any energy production, minifrac, micro-hydraulic fracturing or extended leak-off test are an efficient way to gain knowledge about in-situ stress field of rock near the wellbore. This direct method means pressurizing an isolated interval of an open borehole section until the rock fractures hydraulically. An obvious signature of fracturing and hence fluid leakage into formation is the non-linear behaviour in the pressure versus time relationship, that is, breakdown in the pressure curve.