Shale fracture surface area measured by tracking exchangeable cations

Hydrocarbon production from oil shale and shale gas is increasingly important for securing the energy supply to society. Such shale reservoirs, however, typically have low permeability, and hydraulic fracturing is required to facilitate economic production. Hydraulic fracturing significantly increases fracture-matrix contact areas (through activating pre-existing fractures as well as creating an artificial fracture network), which is of key importance for efficient production. In this context it is vital to estimate this contact area and associated fracture network structures. Conventional techniques, i.e. micro-seismic mapping and pressure transient analysis, however, only deliver limited information. We thus propose here a new experimental technique, which can measure fracture-matrix contact areas; and the accuracy of contact area measurements can be considerably improved. The proposed technique is based on cation exchange processes and chemical tracer measurements. We verified this technique experimentally with laboratory measurements and demonstrated that fracture-matrix areas can be measured with good precision. It is furthermore possible to gather information about the fracture network structure by conducting transient measurements. We conclude that the proposed technique is feasible, and can be combined with conventional techniques to significantly improve measurement accuracy

Brittleness of gas shale reservoirs: A case study from the north Perth basin, Australia

Shale reservoirs have gained the attention of many in recent years due to their potential as a major gas resource. Production from this kind of formation, however, requires an accurate estimation of brittleness and employments of hydraulic fracturing. There have been many studies as to how brittleness can be estimated, but few research works were carried out so far indicating how brittleness indices vary in gas shale formations. The aim of this paper is to evaluate the variation of brittleness in one of the gas shale reservoirs located in the north Perth Basin of Australia. The results obtained indicated that the lower part of the Carynginia shale should be selected for a hydraulic fracturing job due to a high brittleness index, although a careful analysis of Total Organic Content (TOC) might be required before initiating any plans. The mineralogical report and interpretations revealed that the space created by cross-plotting the elastic parameters is able to identify dominant minerals contributing into brittleness. Performing a series of true triaxial tests, which are capable of simulating the real field condition by applying three independent principal stresses, implied that as the stress anisotropy increases, a transition takes place from brittle towards the ductile behaviours. However, when this anisotropy becomes significant, samples regain their strength. This study, therefore, recommends more studies to get a practical conclusion on brittleness under true triaxial conditions

Nanoparticles influence on wetting behaviour of fractured limestone formation

Nanoparticles have gained considerable interest in recent times for oil recovery purposes owing to significant capabilities in wettability alteration of reservoir rocks. Wettability is a key factor controlling displacement efficiency and ultimate recovery of oil. The present study investigates the influence of zirconium (IV) oxide (ZrO2) and nickel (II) oxide (NiO) nanoparticles on the wetting preference of fractured (oil-wet) limestone formations. Wettability was assessed through SEM, AFM and contact angle. The potentials of the nanoparticles to alter oil-wet calcite substrates water wet, was experimentally tested at low nanoparticle concentrations (0.004–0.05 wt%). Quite similar behaviour was observed for both nanoparticles at the same particle concentration; while ZrO2 demonstrated a better efficiency by altering strongly oil-wet (water contact angle θ=152°) calcite substrates into a strongly water-wet (θ=44°) state, NiO changed wettability to an intermediate-wet condition (θ=86°) at 0.05 wt% nanoparticle concentration. We conclude that ZrO2 is very efficient in terms of inducing strong water-wettability; and ZrO2 based nanofluids have a high potential as EOR agents

Permeability evolution in sandstone due to injection of CO2-saturated brine or supercritical CO2 at reservoir conditions

We measured the change in permeability of two selected sandstones (Berea, Fonteinebleau) due to injection of CO2-saturated (“live”) brine, unsaturated (“dead”) brine or supercritical (sc) CO2 at reservoir conditions. We found that the permeability did not significantly change in a clean sandstone consisting of pure quartz (Fonteinemebleau) due to live or dead brine injection, although permeability changed due to scCO2 injection by ~23%. The permeability in the Berea sandstone, however, changed due to live or dead brine injection, by up to 35%; this permeability reduction in Berea sandstone was likely caused by fines release and subsequent pore throat plugging as the damage was more significant at higher injection rates. We expect that this phenomenon – i.e. rock permeability reduction due to CO2 injection into the formation – can have a significant and detrimental influence on CO2 injectivity, which would be reduced accordingly

The role of rock joint frictional strength in the containment of fracture propagation

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities

Experimental Investigation of Hydraulic Fracturing in Vertical and Horizontal Perforated Boreholes

In this study, scaled hydraulic fracturing tests are conducted on 10 and 15 cm cubical mortar samples. The importance of scaled fracturing tests should be highlighted as the results of non-field-like fracturing tests cannot be compared with or used for actual fracturing operation. Three independent principal stresses were applied to the samples using a True Tri-axial Stress Cell (TTSC). The hole and perforations were made into the sample after casting and curing were completed. Various scenarios of vertical and horizontal wells and in-situ stress regimes were modeled. These two factors play a significant role in fracture initiation and near wellbore propagation parameters; however they are not independent from each other and should be analyzed simultaneously. The results showed that when the least stress component is perpendicular to the axis of the perforations, less fracturing pressures would be required. It is also shown that, even when the cement sheath is failed, the orientation of the perforations affects the fracturing process noticeably. Furthermore, it was found that stress anisotropy influences the fracturing mechanism in a perforated borehole, and affects the geometry of the fracture close to the wellbore

Influence of wettability on residual gas trapping and enhanced oil recovery in three-phase flow: a pore-scale analysis using micro-computed tomography

We imaged two enhanced oil recovery processes in a mixed-wet sandstone plug at high resolution (3.4µm)3 with a micro-computed tomograph. In the first process, gas was directly injected into an oil reservoir which was subsequently waterflooded (Sogb process). In the second process, the oil reservoir was first waterflooded, then gas-flooded and finally waterflooded again (Sobgb process – typically referred to as water-alternating-gas process). We qualitatively found that during immiscible gas/brine/oil displacements a) similar amounts of gas can be stored in a mixed-wet reservoir with the Sogb and the Sobgb processes compared with the Sogb process in a water-wet reservoir (note that less gas can be stored in a water-wet reservoir with the Sobgb process), and b) more oil can be produced with a Sobgb process compared to Sogb in a mixed-wet reservoir, contrary to the situation in a water-wet reservoir, where the Sogb process is more efficient. In addition, we identified several pore-scale fluid configurations, which ultimately determine reservoir flow properties