Summary: Hydraulic fracturing is one of the important stimulation methods to enhance the productivity of coalbed methane (CBM) wells. However, the commonly used water-based fracturing fluids can bring some bottlenecks such as large amount of water consumption, clay-mineral swelling, and poor fracturing performance on ductile coals. Cyclic liquid nitrogen (LN2) fracturing, as a novel nonaqueous stimulation method, has the potential to solve the above problems. In cyclic LN2 fracturing, supercooling LN2 is injected in a cyclic manner [i.e., alternating high injection rate (or pressure) and low injection rate (or pressure)]. Coals will be subjected to cyclic freeze-thaw, stress oscillation, and fatigue damage, which is expected to improve the stimulated reservoir volume. First, laboratory cyclic LN2 fracturing tests were conducted on coal samples with various coal ranks to investigate the fracture initiation/propagation behavior and fracture network patterns. Cyclic water fracturing tests were also conducted as comparisons. Then, computed tomography (CT) scanning and geomechanical/petrophysical properties tests before and after LN2 fracturing were performed to assist in understanding the cyclic LN2 fracturing mechanisms and implications. Finally, to solve the field application concerns, we investigated the possible fracture geometries at the field scale, temperature distribution of LN2 along the wellbore during injection, and the economic feasibility. The key factors affecting the temperature distribution during LN2 transportation along the wellbore were clarified for the first time. The results indicate that cyclic LN2 fracturing shows the potential to decrease the breakdown pressure and produce complex fracture networks. Different coal ranks have different responses to cyclic LN2 fracturing attributed to the variances in natural fracture development and geomechanical/petrophysical properties. Besides, increasing the cycle number is effective in enhancing the cyclic LN2 fracturing performance on coals with relatively higher geomechanical strengths and tighter rock mass. The suggested cycle numbers from low to high for different coal ranks are listed here: low-rank coal < high-rank coal < middle-rank coal. In field applications, gaseous nitrogen (N2) can be used as the annulus fluid to provide an effective insulation for heat transfer between the low-temperature LN2 and the surrounding environment. In addition, the net present value (NPV) analysis indicates that LN2 fracturing is an economically feasible stimulation method, which can exceed slickwater fracturing in some cases. The key findings are expected to provide preliminary insights into the potential field applications of cyclic LN2 fracturing in CBM or other unconventional oil/gas exploitation.

An overview of wellbore methods of investigating the stress-strain state of the upper layers of the Earth’s crust is given in the paper. Some main methods of stress estimation from wellbore...

The paper is devoted to the problem of reconstructing stress state of the rock mass using data on natural fractures. A particular problem of reconstructing stress profiles along the well trajectory from data on spatial orientations and activity of natural fractures in well surrounding rock masses is considered. An approach for estimation of tectonic stresses from these data developed in the previous studies is considered with regard to the specifics of inverse problem solution. An optimization problem emerging during stress reconstruction procedure is stated and various methods of its solution are analyzed. Four different ways to define the objective functions measuring the degree of agreement between real and modeled fractures properties which can be used to reconstruct stresses based on natural fractures related data. The effect of objective function definition on the inverse problem solution is studied in a comparative way. To do that, a synthetic fracture model is constructed; the problem of rock mass stress state reconstruction is stated and solved for the synthetic model using different objective functions. It is revealed that variation of objective function used in practice for stress state reconstruction from natural fractures data leads both to alteration of the obtained solution for the inverse problem and solution uniqueness and stability with regard to worsening of the input data. The paper presents certain conclusions related to the suggestions on choosing a particular objective function for inverse problem solution depending on presence and quality of data related to natural fractures.