Evaluation Method of the Vertical Well Hydraulic Fracturing Effect Based on Production Data

Hydraulic fracturing technology has become a key technology for the development of low-permeability/tight oil and gas reservoirs. The evaluation on the postfracturing effect is imperative to the formulation and implementation of the fracturing and development plan. Based on the characteristics of the flow in fracture network after a large-scale hydraulic fracturing, a numerical method for evaluating the effect of fracturing in vertical well was established. This study conducts postfracturing effect evaluations to block C Oilfield’s wells that underwent conventional fracturing and volumetric fracturing, respectively, proposes the definition of fracture network conductivity and its relationship with cumulative production, and analyzes the fracturing construction parameters. The results suggest that the conventional fracturing can only form a single fracture instead of a stimulated reservoir volume (SRV) region. However, the volumetric fracturing transformation can form a complex fracture network system and SRV region and meanwhile bring obvious increase in the production. The effective time lasts for a longer period, and the increase of average daily oil is 2.2 times more than that of conventional fracturing. Additionally, with the progress of the production, the SRV area within the core region of the volume transformation gradually decreased from 6664.84 m2 to 4414.45 m2; the SRV area of the outer region decreased from 7913.5 m2 to 5391.3 m2. As the progress develops, the equivalent permeability and the area of the fracture gradually decrease as the fracturing effect gradually weakens, and so does the conductivity of the network decreasing exponentially; a good correlation is observed between the conductivity of the fracture network, the cumulative production, and fracturing construction parameters, which can serve as the evaluation parameters for the fracturing effects and the basis for fracturing productivity prediction and provide a guidance for fracturing optimization design.

Structural and dynamic conditions for the formation of large orogenic gold deposits in Central and Northeast Asia

Research subject. Large orogenic gold deposits in the fold belts of Central and Northeast Asia.Materials and methods. Geological mapping of various scales on a number of large orogenic gold deposits was conducted using the methods of structural-paragenetic analysis of metamorphic strata, accompanied by obligatory linking of ore mineralization manifestations to specific structures. In a number of cases, various statistical methods were used to geometrize mineralization, identify patterns of its location and determine the paths of paleofluid flows. Available publications on the objects under consideration were reviewed. The geological and structural features of large orogenic gold deposits – Muruntau, Kokpatas, Sukhoi Log and Pavlik – were considered.Results. The Muruntau, Kokpatas and the Sukhoi Log ore deposits are of shariyage-thrust type. Compared to these objects, the Pavlik field is confined to a zone of volumetric fracturing between a series of reverse faults, feathering a large deep fault and belonging to the transpression type. At the Muruntau and Pavlik deposits, the analysis of the location of the most intensive mineralization substantiated the paths of paleofluid flows, along which the fluid migration and ore formation took place.Conclusions. The distribution of ore mineralization in the Muruntau deposit obeys the orientation of planar (cleavage) and linear (orientation of fold hinges) elements. Apparently, the former (main) direction may indicate the orientation and position of the main migration route of ore-bearing fluids, while the latter corresponds to secondary channels, the position of which is due to the intersection of syn-napping structures with favourable lithological horizons. For the Pavlik deposit, the position of ore pillars is compared with the paths of paleofluid flows, the root parts of which are promising for identifying the most powerful and intense mineralization.

Experimental Study of Volumetric Fracturing Properties for Shale under Different Stress States

Shale gas can be commercially produced using the stimulated reservoir volume (SRV) with multistage fracturing or multiwell synchronous fracturing. These fracturing technologies can produce additional stress fields that significantly influence the crack initiation pressure and the formation of an effective fracture network. Therefore, this study primarily investigated the evolution of crack initiation and propagation in a hydraulic rock mass under various stress conditions. Combining the in situ stress characteristics of a shale reservoir and fracturing technology, three types of true triaxial volumetric fracturing simulation experiments were designed and performed on shale, including three-dimensional constant loading, one-dimensional pressurization disturbance, and one-dimensional depressurization disturbance. The results indicate that the critical failure strength of the shale rock increases as the three-dimensional constant loads are increased. The rupture surface is always parallel to the maximum principal stress plane in both the simulated vertical and horizontal wells. Under the same in situ stress conditions in the wellbore direction, if the lateral pressure becomes larger, the critical failure strength of shale rock would increase. Additionally, when the lateral in situ stress difference coefficient is smaller, the rock specimen has an evident trend to form more complex cracks. When the shale rock was subjected to lateral disturbance loads, the critical failure strength was approximately 10 MPa less than that in the state of constant loading, indicating that the specimen with disturbance loads is more likely to be fractured. Moreover, shale rock under the depressurization disturbance load is more easily fractured compared with the pressurization disturbance. These findings could provide a theoretical basis and technical support for multistage or multiwell synchronous fracturing in shale gas production.

Correction to: Mechanical and Volumetric Fracturing Behaviour of Three‑Dimensional Printing Rock‑like Samples Under Dynamic Loading

The article Mechanical and Volumetric Fracturing Behaviour of Three-Dimensional Printing Rock-like Samples.

Mechanical and Volumetric Fracturing Behaviour of Three-Dimensional Printing Rock-like Samples Under Dynamic Loading

Heterogeneous rock contains numerous pre-existing three-dimensional (3D) cracks, which control its mechanical and fracturing properties. Considerable effort has been devoted to studying the volumetric fracturing behaviour of rock under static loading conditions. Although rock masses are often subject to dynamic impacts such as earthquakes and blasting, the mechanical and volumetric fracturing behaviour of rock under dynamic loading is still poorly understood. In this paper, dynamic laboratory tests were performed on 3D-printed artificial rock samples with 3D embedded flaws created during three-dimensional printing (3DP), with the aim of studying the volumetric fracturing and mechanical properties of these samples under impact with high strain rate. The results show that the dynamic compressive strength and the tangent modulus decrease with an increasing number of flaws, but have very limited effects on the ratio of the fracture initiation stress of the first crack to the peak stress of the sample, the maximum axial strain of the sample and the volumetric fracturing behaviour of the sample. The tensile failure of a sample is caused by the continuous extension of wing cracks from the outer flaw tips. The mechanical and volumetric fracturing behaviour of samples with 3D embedded flaws are strain rate dependent. The tangential modulus and the ratio of the fracture initiation stress of the crack to the peak stress increase significantly when the loading type changes from static compression to dynamic compression. Under dynamic compression, wing cracks can continuously extend to the sample ends, whereas under static compression, wing cracks can intermittently extend only a limited distance. Moreover, the fracturing behaviour of 3D flaw differs from that of 2D flaws under dynamic loading. Under high strain rate loading, wing cracks generated at 3D flaw tips lead to splitting failure of the sample, while shear cracks formed at 2D flaw tips result predominant shear failure of the sample. The findings in this paper could facilitate a better understanding of rock failure subjected to dynamic loading conditions.

Study on reasonable nozzle size after volumetric fracturing of glutenite reservoir

What kind of drainage system should be adopted after the volumetric fracturing of the glutenite reservoir in the Mahu 1 well area, lack of systematic research. The reasonable choice of the size of the nozzle is the key to determining the flow back of the fracturing fluid and stable production. By comparing the applicability differences of fracturing and drainage systems at home and abroad, this paper analyzes the production experience of neighboring areas and determines the overall principles of drainage system: small displacement principle, stable drainage principle and step-by-step amplification principle. Then, by calculating the rate of change of oil production, analyzing the system well test curve and production curve, and determining a reasonable drainage system, it is concluded that the volume of the glutenite reservoir after fracturing should be 1.5 mm small nozzle, and the pressure is gradually stabilized. The grade is replaced by a 2.5 mm nozzle. Most of the wells are finally stabilized in the 3.0 mm nozzle production. A small number of wells can be properly enlarged to a 3.5mm nozzle for stable production. Under the guidance of this understanding, the horizontal wells in the Mahu 1 well area have basically reached the design capacity, providing theoretical basis and guidance for the production practice after volume fracturing.

Failure analysis of long round thread in horizontal well casing under multi-axis loading

The casing is subjected to complicated forces underground, and the threaded joint of the casing is the weak link of the casing. becoming more and more severe, various kinds of failure accidents often occur in practical use. Therefore, in view of the casing thread fracture failure during the process of volumetric fracturing in well W of an oil field. The finite element model of 5-1/2"API casing long round threaded joint was established in this paper, ABAQUS software was used to simulate and analyze the stress and deformation of casing thread under the loading state of overlock, axial tension and pressure, and fracturing internal pressure. The results show that the stress distribution of teeth is reasonable. Under the condition of axial tension and compression, the maximum stress of casing thread exceeds the yield strength into plasticity and causes damage. However, when fracturing and stimulation technology is implemented, the stress of the collar and casing body increases significantly, and the fracture is caused by fatigue and extended fracture under the alternating fracturing load. The finite element analysis results are consistent with the field failure results. Study the influence of downhole complex working condition on casing thread by simulation, which is of great significance to the protective casing.