Journal of Unconventional Oil and Gas Resources最新文献

The most effective method for stimulating unconventional reservoirs is using properly designed and successfully implemented hydraulic fracture treatments. The interaction between pre-existing natural fractures and the engineered propagating hydraulic fracture is a critical factor affecting the complex fracture network. However, many existing numerical simulators use simplified model to either ignore or not fully consider the significant impact of pre-existing fractures on hydraulic fracture propagation. Pursuing development of numerical models that can accurately characterize propagation of hydraulic fractures in naturally fractured formations is important to better understand their behavior and optimize their performance.

In this paper, an innovative and efficient modeling approach was developed and implemented which enabled integrated simulation of hydraulic fracture network propagation, interactions between hydraulic fractures and pre-existing natural fractures, fracture fluid leakoff and fluid flow in reservoir. This improves stability and convergence, and increases accuracy, and computational speed. Computing time of one stage treatment with a personal computer is now reduced to 2.2 min from 12.5 min than using single porosity model.

Parametric studies were then conducted to quantify the effect of horizontal differential stress, natural fracture spacing (the density of pre-existing fractures), matrix permeability and fracture fluid viscosity on the geometry of the hydraulic fracture network. Using the knowledge learned from the parametric studies, the fracture–reservoir contact area is investigated and the method to increase this factor is suggested. This new knowledge helps us understand and improve the stimulation of naturally fractured unconventional reservoirs.

For rational depth modeling of a prominent reservoir layer in north of Iran (Gorgan plain, Chelekan top), geostatistical methods were proposed to use with the minimum available data. This data consisted of ten wells, five 2D seismic lines (three vertical lines perpendicular to two horizontal ones) which covers the area, and one small 3D seismic area, which was applied solely for evaluation of findings and optimizing our choices. Because the expansion of this area was limited as opposed to region aimed for modeling. Hence, for a reasonable geostatistical modeling, an appropriate secondary variable (soft data) was crucial. Initially, the reservoir layer should be pursued in five seismic lines with a suitable seismic attribute and achieved its time model (TWT) all over the Gorgan plain due to existing a few number of lines, linear form of data set (located on the seismic lines) and the smoothing effect of kriging, the estimate and average simulated realizations (E-type) could not give acceptable results in time modeling of the layer based on merely five seismic lines. Therefore, one of 100 realizations related to sequential quassian simulation (SGS) selected as the best secondary data after probing their correlation and similarity with the real 3D seismic data and obtaining a proper correlation coefficient. Moreover, this realization revealed the best correlation with the depth amounts of 10 wells, reproducing geostatistical and statistical parameters of input data. For this reason, it was utilized as secondary data in kriging with an external drift method (KED). Having been applied it, the smoothing effect was diminished dramatically in comparison with one variable model and consequences of final modeling, investigation of uncertainty and estimate error prior to using secondary data and after that, all of them signified the final model was much more reasonable than initial one (without secondary data).

The Perth Basin is one of the major tectonic structures along the western continental margin of Australia and was initially formed through the rifting and break-up of the Indian and Australian plates. The severe tectonic movements accompanied and occurred after the break-up are responsible for the most structural elements and for the distribution of pore pressure in the basin.

Investigations on the well log data from the Perth Basin have identified shale intervals which are characterised as overpressured in some parts of the basin, whereas similar shale intervals found to be normally pressured in other parts of the basin. The phenomena of overpressure have frequently been reported while drilling the same intervals. Based on this research, sections with overpressure were observed in the majority of the wells in the basal section of the Kockatea shale where there were less tectonic activities have been recorded. Normal pore pressure was observed in shallower wells in the Kockatea shales which were located within uplifted sections that were more tectonically active areas.

Based on the results of this research, the pore pressure distribution in the Kockatea Shale varied significantly from one part of the Perth Basin to another as a result of compressive tectonic stress. Compressional tectonic activities either induced fracturing in shallower localities (e.g. Beagle Ridge, Cadda Terrace and the adjacent terraces) or removed part of the Kockatea Shale as a result of faulting resulting in overpressures being released. Regions with less intensity of the tectonic activities showed an increase in pressure gradients as approaching away from the centre of uplift.

The well cement used in injection/production wells plays a major role in the success of a carbon capture and storage project. Ordinary Portland cement (OPC)-based well cement has been used in injection/production wells and it has been found to be unstable in CO₂-rich environments. In recent times, geopolymers have been tested as an alternative to OPC, and it has been found that geopolymers perform better than OPC under CO₂-rich down-hole conditions. In this research work, a numerical study was performed to model CO₂ flow through geopolymer under down-hole stress conditions using COMSOL multiphysics. First, the model was validated using experimental flow results under drained triaxial conditions for various injection and confining pressures. The model was then extended to predict the flow characteristics such as permeability, Darcy’s velocity, CO₂ pressure and CO₂ concentration distributions in geopolymer under high injection and confining pressures. The CO₂ permeability values predicted by the model were in good agreement with the experimental permeability values for various injection (3–13 MPa) and confining pressures (10–25 MPa). The CO₂ permeability of geopolymer varies between 0.008 and 0.014 μD for injection pressures of 15–40 MPa and confining pressures of 30–45 MPa. The flow parameters including Darcy’s velocity, CO₂ pressure and CO₂ concentration in geopolymer reduces with increase in confining pressures due to the reduction of pore volume with increase in confinement. Pressure-driven advection is the dominant CO₂ transport mechanism during the injection period compared to concentration-driven diffusion. CO₂ transport through geopolymer can be modelled using COMSOL multiphysics.

Detailed analysis of the organic matter, mineralogy, and related rock properties of the sediments of the Neal shale member of the Floyd shale group in the Black Warrior Basin were done to determine the cause of the lack of adequate production in this shale gas play. Analysis of pilot well cores found the organic-richness, kerogen type, maturity, thickness, porosity/permeability, and geomechanical behavior were all found to be satisfactory for a potential shale play. Although bulk mineralogy compared favorably with other shale plays, some of the testing pointed toward fluid–clay interactions and proppant embedment as the cause for the lack of production in this shale gas play. However, close proximity to gas charged overlying sandstones along with normal pressure in this shale reservoir suggest potential seal problems have reduced the gas charge in the shale. This led to changes in the screening parameters for new plays, emphasized the importance of doing look backs on failed projects, and the need to integrate learnings into future project evaluations.

More than 2 × 10⁴ m³ of water containing additives is commonly injected into a typical horizontal well in gas shale to open fractures and allow gas recovery. Less than half of this treatment water is recovered as flowback or later production brine, and in many cases recovery is <30%. While recovered treatment water is safely managed at the surface, the water left in place, called residual treatment water (RTW), slips beyond the control of engineers. Some have suggested that this RTW poses a long term and serious risk to shallow aquifers by virtue of being free water that can flow upward along natural pathways, mainly fractures and faults. These concerns are based on single phase Darcy Law physics which is not appropriate when gas and water are both present. In addition, the combined volume of the RTW and the initial brine in gas shale is too small to impact near surface aquifers even if it could escape. When capillary and osmotic forces are considered, there are no forces propelling the RTW upward from gas shale along natural pathways. The physics dominating these processes ensure that capillary and osmotic forces both propel the RTW into the matrix of the shale, thus permanently sequestering it. Furthermore, contrary to the suggestion that hydraulic fracturing could accelerate brine escape and make near surface aquifer contamination more likely, hydraulic fracturing and gas recovery will actually reduce this risk. We demonstrate this in a series of STP counter-current imbibition experiments on cuttings recovered from the Union Springs Member of the Marcellus gas shale in Pennsylvania and on core plugs of Haynesville gas shale from NW Louisiana.

The imbibition of fracturing fluid into the shale matrix is identified as one of the possible mechanisms leading to high volumes of water loss to the formation in hydraulically fractured shale reservoirs. In an earlier study (Makhanov et al, 2012), several spontaneous imbibition experiments were conducted using actual shale core samples collected from Fort Simpson, Muskwa and Otter Park formations, all belonging to the Horn River shale basin. This study provides additional experimental data on how imbibition rate depends on type and concentration of salt, surfactants, viscosifiers and sample orientation with regard to the bedding plane. The study also proposes and applies a simple methodology to scale up the laboratory data for field-scale predictions.

The data show that an anionic surfactant reduces the imbibition rate due to the surface tension reduction. The imbibition rate is even further reduced when KCl salt is added to the surfactant solution. Surprisingly, viscous XG solutions show a considerable spontaneous imbibition rate when exposed to organic shales, although their viscosity is much higher than water viscosity. This observation indicates that water uptake of clay-rich organic shales is mainly controlled through preferential adsorption of water molecules by the clay particles, and high bulk viscosity of the polymer solution can only partly reduce the rate of water uptake.

The field scale calculations show that water loss due to the spontaneous imbibition during the shut-in period is a strong function of fluid/shale properties, fracture-matrix interface, and soaking time. The presented data and analyses can be used to explain why some fractured horizontal wells completed in gas shales show poor water recovery and an immediate gas production after extended shut-in periods.