The effect of saturation conditions on fracture performance of different soundless cracking demolition agents (SCDAs) in geological reservoir rock formations

Highlights

• Multiplanar radial fractures produced during SCDA charging.

• Pore-water in rock decreases the fracture performance of SCDAs.

• Saline pore-fluids (NaCl) increases the fracture performance of SCDA.

• Reaction rate of SCDA is dependent on the salinity of the rock pore-fluid.

• Orientation of generated fractures using SCDA varies with different pore-fluids.

Abstract

Fracture stimulation using soundless cracking demolition agents (SCDAs) is a potential alternative technique to induce high-density fractures in sedimentary reservoir-rock as an auxiliary technique to improve the efficiency of enhanced oil and gas recovery efficiencies. However, to date, its application has been limited to fracture stimulation in dry rock masses. Therefore, using modified SCDAs, which can be used for underwater rock fracturing, a series of experiments was conducted to investigate the fracturing performance of SCDAs in saturated rock masses. 18 coarse-grained sandstone specimens were saturated in water, oil, and NaCl brine and fractured using three different SCDA types: a standard SCDA (S1), and two modified for underwater application (S2) and accelerated reaction rate (S3). Then, the fractured samples were scanned in the Australian Synchrotron, and the fractures were quantified using Avizo 9.0.1. The fracture initiation time and the total fracture network length and volume were found to be dependent on the saturated pore fluid of rock. Water saturation of samples increased the fracture initiation time by 16.5%, 24.1% and 13.68% for S1, S2, and S3 type SCDAs respectively and reduced the fracturing potential of SCDA by 59.5%, 32.49% and 66.67% compared to dry samples. This reduction was less apparent in oil-saturated samples as the high pore fluid viscosity of oil-saturated samples aid fracturing, which is explained by the Poiseuille equation. Increasing salinity in the saturation fluid from 0% to 12.5% was favourable for the fracturing efficiency of SCDAs because of the formation of CaCl₂ in the pore fluid, which accelerates the reaction of SCDA. Fracture orientation also changed depending on the saturation fluid, which was again governed by the variation in reaction rate in SCDAs under different saturation conditions.

Introduction

With the increasing depletion of conventional reservoirs, low-permeability reservoirs have become a viable option for oil and gas recovery (Adibhatla and Mohanty, 2008; You et al., 2018). This depletion is driven by the excessive energy demand, and the global energy consumption is estimated to be nearly doubled in 2030 compared to 1980 (Weissler, 2008). Techniques such as artificial fracture stimulation by hydraulic fracturing supplemented by horizontal wells (Dehghanpour et al., 2012; Zhao et al., 2016; Wang et al., 2018) is required for low permeability reservoir fracture stimulation to improve the production efficiency of enhanced oil recovery (EOR) systems. Hydraulic fracturing typically results in a single planar fracture emanating from the well (Weng et al., 2011; Wanniarachchi et al., 2018) and the fracture density of the reservoir rock can be improved by multistage fracturing where, a reservoir rock is fractured in multiple successions along the horizontal well (Hejl et al., 2006; Bello and Wattenbarger, 2010). However, the excessive utilization of fresh water, unidirectional fracturing and the presence of many chemical additives lead to high costs and environmental pollution (Chang et al., 2014; You et al., 2018), which makes multi-staged fracturing uneconomical. Therefore, it is crucial to develop alternative fracture stimulation technologies to improve the efficiency of energy recovery. In this light, the development of alternative mining technologies must be focused on reducing the environmental impact of conventional fracture stimulation methods.

Soundless cracking demolition agents (SCDAs) have been identified as a viable option to produce controlled fractures in a target rock to enhance the permeability of the rock mass (DE Silva et al., 2016). SCDA is a calcium oxide based cementitious compound, which expands volumetrically when hydrated. When SCDA is injected into a reservoir rock through a pre-drilled injection well, the volumetric expansion produces an expansive pressure within the injection well. When the expansive pressure exceeds the tensile strength of the rock, it produces multiple radially propagating fractures from the injection well. Volumetric expansion of SCDA generates tensile hoop stresses around the injection well (Harada et al., 1989) and the extent of fracture growth is controlled by controlling the tensile stress field around the injection well, which can be altered with the dose of SCDA injected. Therefore, the injection of SCDA can initiate multi-directional fractures around the well bore, which can then be extended using existing methods such as hydraulic fracturing. However, the application of SCDA has been limited to rock fragmentation in dry rocks (Arshadnejad et al., 2011), tunnelling works (Tang et al., 2017) and demolition works (Gambatese, 2003; Natanzi et al., 2016) due to the dissolution of SCDA in deep saturated rock masses. However, a recent development in SCDA has enabled the use of SCDA in deep saturated rock masses by incorporating hydrophobic properties to SCDA (DE Silva et al., 2018d).

The application of SCDA can be extended to induce fractures in sedimentary basins as an auxiliary fracturing technique to improve EOR efficiency. EOR techniques, which include miscible gas injection (Jia et al., 2017), water alternating gas (Awan et al., 2008) and chemical flooding (Zhang et al., 2010), all suffer from limited reservoir rock permeability (Muggeridge et al., 2014). Inducing a fracture network in the target rock by SCDA injection prior to recovery operations produces multi-planar fractures around the injection well as opposed to single planar fractures in hydraulic fracturing. Supplementing hydraulic fracturing with SCDA charging could potentially eliminate the need for multistage fracturing as multiple radial fractures are formed around the wellbore during SCDA charging. The effect of geo-stresses and hydrostatic pressures on the fracture performance of SCDA has been investigated (DE Silva et al., 2018a; De Silva et al., 2018e). However, the fracturing performance of the modified SCDAs under different saturation conditions of the reservoir rock has not been explored previously. This is an important aspect that needs to be investigated because the hydration reaction of SCDA can be influenced by the pore fluid of the reservoir rock. Furthermore, SCDAs have been identified to broaden the application range of other applications such as mineral extraction in relatively impervious rocks by inducing a fracture network in a target rock. Therefore, the aim of this paper is to investigate the influence of different pore fluids in a reservoir rock on the performance of SCDA-assisted reservoir fracturing prior to any large-scale application.