Rock Mechanical Properties and Breakdown Pressure of High-Temperature and High-Pressure Reservoirs in the Southern Margin of Junggar Basin

The mechanical properties of the high-temperature and high-pressure reservoirs in the southern margin of Junggar Basin have not been clearly understood, which correspondingly results in uncertainties when predicting the breakdown pressure. To address this issue, firstly, rock mechanical experiments under high temperature, high confining pressure, and high pore pressure were carried out. Secondly, empirical formulas related to the transformation of dynamic and static mechanical parameters in the regional strata were proposed. Finally, the existing prediction model for the formation breakdown pressure was improved by taking the wellbore seepage and thermal stress into consideration. Results show that under the reservoir condition of high temperature and high pressure, the rock sample tends to form closed shear cracks. High temperature causes thermal damages and the reduction of the compressive strength and elastic modulus, while the combined effects of high confining pressure and pore pressure enhance the compressive strength and plasticity of the rock sample simultaneously. Based on the correlation analysis, it is found that the static elastic modulus is linearly related to the dynamic value, while static Poisson’s ratio is a quadratic function of the dynamic value. These fitting functions can be used to obtain the profiles of static elastic modulus and Poisson’s ratio based on their dynamic values from the logging interpretation. Besides, the improved prediction model for the rock breakdown pressure can yield more accurate results indicated by the error less than 2%. Therefore, the proposed breakdown pressure prediction model in this study can provide theoretical guidance in the selection of fracturing truck groups and the design of the pumping schedule for high-temperature and high-pressure reservoirs.

Development of Fracturing Technology for Deep Shale Gas in South Sichuan, China

Under the condition of high ambient temperature and high confining pressure，the physical & mechanical properties and in-situ stress state of deep shale will change noticeably. Normally, the deep-shale formation has high horizontal stress difference (about 11∼21 MPa, 1595∼3045 psi), high fracture-closure pressure gradient (about 0.023∼0.025 MPa/m, 1.017∼1.105 psi/ft), high breakdown pressure gradient (larger than 0.03 MPa/m, 1.327 psi/ft), low mechanical brittleness (about 42%∼55%), low difference between the vertical and the horizontal stresses (about 3∼5MPa, 435∼725 psi). The complex geological characteristics of deep shale increase the difficulity of fracturing: 1) effect of brittle/ductile transition under high confining pressure; 2) non-uniform propagation of multi-cluster fractures is more prominent; 3) the migration of proppant is difficult in narrow fracture network; 4) high friction and high pumping pressure; 5) more stringent requirements for fracturing tools; 6) high requirements for fracturing scale, efficiency and economy. To address above challenges, this paper presents a comprehensive overview of latest researching and applicable techniques about deep-shale fracturing (3500<TVD<3800 m, 11482∼12467 ft), including: 1) new evaluation methods on fractured shale quality and fracability, considering vertical stress difference coefficient and effective confining stress; 2) non-uniform propagation of fractures in multi-clusters perforation; 3) reveal the transport mechanism of proppant in narrow fracture network; 4) optimization of high performance fracturing fluid systems to enlarge the ESRV in deep shale; 5) development of a new staged fracturing tool for deep-shale fracturing, including dissoluble bridge plug and toe delayed sleeve; 6) an integrated geoscience and engineering simulation to optimize the treatment parameters and to achieve the best fracturing efficiency in the deep shale strata. The hydraulic fracturing technique for deep shale gas with the depth of 3500∼4500 m (11482∼14763 ft) has formed preliminarily. The hydraulic fracturing technology for deep shale gas (TVD≥3500∼3800 m, 11482∼12467 ft) have made a breakthrough in Sichuan basin, China, and significant progress has also made in 3800-4500m TVD (12467∼14763 ft). The research results and techniques introduced in the paper have been successfully applied to more than 100 wells in the Sichuan basin. The test production of part fractured well can reach (10∼31)×104 m3 per day (0.35∼1.09×107 SCF/day), which basically realizes the economical and effective development for deep shale gas.

The Undrained Characteristics of Tengger Desert Sand from True Triaxial Testing

Aimed at the characteristics of aeolian sand under rapid construction conditions in desert geotechnical engineering, a series of the true triaxial undrained test were carried out on the GDS apparatus. The 3D deformation, failure, and other characteristics of the dense sand are obtained. Under the condition of same p c , the state transition point where the void water pressure changes from increasing to decreasing appears earlier and leads to enhanced dilatancy with the increase of b, which means the enhanced dilatancy of dense sand caused the increase in strength. The results of the same b shows that the void water pressure generally indicates a decrease at low confining pressure and an increase at high confining pressure, indicating that the aeolian sand shows dilatancy at low confining pressure and contraction at high confining pressure. The state transition point increases with the increase of p c , but all points tend to the same critical state line and state transition line. When b = 0, the critical state line is q = 1.57 p ′ , and the state transition line is q = 1.23 p ′ . When b = 1, the critical state line is q = 1.24 p ′ , and the state transition line is q = 1.04 p ′ . The results at same b obtained the unified critical state line and the state transition line. Therefore, the true triaxial test can obtain the unified relationship of void ratio, p c and b, which overcomes the fact that the existing test cannot consider the influence of b. The test results provide a basis data for the design, construction, and maintenance of geotechnical engineering in Tengger Desert.

Numerical simulation of hydraulic fracturing in enhanced geothermal systems considering thermal stress cracks

With the increasing attention to clean and economical energy resources, geothermal energy and enhanced geothermal systems (EGS) have gained much importance. For the efficient development of deep geothermal reservoirs, it is crucial to understand the mechanical behavior of reservoir rock and its interaction with injected fluid under high temperature and high confining pressure environments. In the present study, we develop a novel numerical scheme based on the distinct element method (DEM) to simulate the failure behavior of rock by considering the influence of thermal stress cracks and high confining pressure for EGS. We validated the proposing method by comparing our numerical results with experimental laboratory results of uniaxial compression tests under various temperatures and biaxial compression tests under different confining pressure regarding failure patterns and stress-strain curves. We then apply the developed scheme to the hydraulic fracturing simulations under various temperatures, confining pressure, and injection fluid conditions. Our numerical results indicate that the number of hydraulic cracks is proportional to the temperature. At a high temperature and low confining pressure environment, a complex crack network with large crack width can be observed, whereas the generation of the micro cracks is suppressed in high confining pressure conditions. In addition, high-viscosity injection fluid tends to induce more hydraulic fractures. Since the fracture network in the geothermal reservoir is an essential factor for the efficient production of geothermal energy, the combination of the above factors should be considered in hydraulic fracturing treatment in EGS.

Shaft Resistance of Long (Flexible) Piles Considering Strength Degradation

Soil-structure frictional resistance is an important parameter in the design of many foundation systems. The soil-structure interface area is responsible for load transferring from the structure to the surrounding soil. The mobilized shaft resistance of axially loaded, long slender pile embedded in dense, dry sand is experimentally and numerically analyzed when subjected to pullout force. Experimental setup including an instrumented model pile while the finite element method is used as a numerical analysis tool. The hypoplasticity model is used to model the soil adjacent to and surrounding the pile by using ABAQUS FEA (6.17.1). The soil-structure interface behavior depends on many factors, but mainly on the interface soil's tendency to contract or dilate under shearing conditions. To investigate this tendency, three piles with different surface roughness and under different confining pressures are used. A dilation behavior is observed in the relation of the average shaft resistance with the axial displacement for piles with rough and medium roughness surfaces, while contraction behavior is noticed when shearing piles with smooth surfaces. A large shear strength degradation of about (10%) reduction in the shaft resistance is observed under low confining pressure compared to a lesser reduction value of about (2%) under high confining pressure. Good agreement is obtained between the experimental and the numerical results.

A novel triaxial apparatus to evaluate the effect of high temperature nitrogen injection concept and design

High temperature nitrogen, injection into coal seams is supposed to improve the per?meability and thus maintain the safety of underground mining. A novel triaxial appa?ratus is recently developed, aiming at providing the effective method to evaluate the effect of high temperature nitrogen injection. The main feature of this novel appara?tus is its high confining pressure, gas injection with high pressure as well as the high temperature. This new device can be either used for natural coal samples (e.g. intact or fractured) or the synthetic coal samples. A series of leakage tests were conducted to verify the feasibility of this instrument, the results of which have confirmed that the maximum pressure (i.e. 10 MPa) can be reached. In addition, the high temperature and pressure of nitrogen gas can also be sustain at the requested level. Based on the preliminary tests on the instrument, a large amount of tests were carried out to eval?uate the effect of nitrogen injection in enhancing the permeability of coking coal from the Pingdingshan coalfield, China, and the influence of high temperature nitrogen injection on mechanical parameters of coal was obtained.