Jingen Deng

Borehole Stability in High-Temperature Formations

In oil and gas drilling or geothermal well drilling, the temperature difference between the drilling fluid and formation will lead to an apparent temperature change around the borehole, which will influence the stress state around the borehole and tend to cause borehole instability in high geothermal gradient formations. The thermal effect is usually not considered as a factor in most of the conventional borehole stability models. In this research, in order to solve the borehole instability in high-temperature formations, a calculation model of the temperature field around the borehole during drilling is established. The effects of drilling fluid circulation, drilling fluid density, and mud displacement on the temperature field are analyzed. Besides these effects, the effect of temperature change on the stress around the borehole is analyzed based on thermoelasticity theory. In addition, the relationships between temperature and strength of four types of rocks are respectively established based on experimental results, and thermal expansion coefficients are also tested. On this basis, a borehole stability model is established considering thermal effects and the effect of temperature change on borehole stability is also analyzed. The results show that the fracture pressure and collapse pressure will both increase as the temperature of borehole rises, and vice versa. The fracture pressure is more sensitive to temperature. Temperature has different effects on collapse pressures due to different lithological characters; however, the variation of fracture pressure is unrelated to lithology. The research results can provide a reference for the design of drilling fluid density in high-temperature wells.

Mechanical Properties of Gas Shale During Drilling Operations

The mechanical properties of gas shale significantly affect the designs of drilling, completion, and hydraulic fracturing treatments. In this paper, the microstructure characteristics of gas shale from southern China containing up to 45.1% clay were analyzed using a scanning electron microscope. The gas shale samples feature strongly anisotropic characteristics and well-developed bedding planes. Their strength is controlled by the strength of both the matrix and the bedding planes. Conventional triaxial tests and direct shear tests are further used to study the chemical effects of drilling fluids on the strength of shale matrix and bedding planes, respectively. The results show that the drilling fluid has a much larger impact on the strength of the bedding plane than that of the shale matrix. The impact of water-based mud (WBM) is much larger compared with oil-based mud. Furthermore, the borehole collapse pressure of shale gas wells considering the effects of drilling fluids are analyzed. The results show that the collapse pressure increases gradually with the increase of drilling time, especially for WBM.

The Optimization of Sand Control Completion in Bohai Bay

Abstract The application of moderate sand control technology in the area of Bohai Bay has achieved significant success in recent years, but the problems that some wells receive high sand production rate and the sand is uncontrollable still exist. As a result, how to choose a reasonable way of sand control becomes urgent when it comes to field work. Analyzing the design basis of domestic and international sand control method and combining with an indoor large-scale model experiment of sand production, the authors proposes the theoretical basis for the selection of moderate sand control technique: three main parameters as uniformity coefficient (UC), shale content (Vsh), and water swelling property of clay mineral. The design of sand retention precision is mainly according to median grain diameter (D50) of formation sand. On the basis of analyzing the reservoir characteristics and evaluating the productivity under different sand control methods for the typical oil fields in Bohai Bay, the authors present selection criteria of sand control methods for the application of moderate sand control techniques in Bohai Bay, and build a corresponding selection chart. This chart has been applied in the sand control design of Ken Li (KL) oil field in Bohai Bay and has been proved reasonable after indoor test, which shows that this chart can be used in field.

Numerical simulation of mechanical compaction of deepwater shallow sediments

To study the compaction law and overpressure evolution in deepwater shallow sediments, a large-strain compaction model that considers material nonlinearity and moving boundary is formulated. The model considers the dependence of permeability and material properties on void ratio. The modified Cam-Clay model is selected as the constitutive relations of the sediments, and the deactivation/reactivation method is used to capture the moving top surface during the deposition process. A one-dimensional model is used to study the compaction law of the shallow sediments. Results show that the settlement of the shallow sediments is large under their own weight during compaction. The void ratio decreases strictly with burial depth and decreases more quickly near the seafloor than in the deeper layers. The generation of abnormal pressure in the shallow flow sands is closely related to the compaction law of shallow sediments. The two main factors that affect the generation of overpressure in the sands are deposition rate and permeability of overlying clay sediments. Overpressure increases with an increase in deposition rate and a decrease in the permeability of the overlying clay sediment. Moreover, an upper limit for the overpressure exists. A two-dimensional model is used to study the differential compaction of the shallow sediments. The pore pressure will still increase due to the inflow of the pore fluid from the neighboring clay sediment even though the deposition process is interrupted.

Leak-off mechanism and pressure prediction for shallow sediments in deepwater drilling

Deepwater sediments are prone to loss circulation in drilling due to a low overburden gradient. How to predict the magnitude of leak-off pressure more accurately is an important issue in the protection of drilling safety and the reduction of drilling cost in deep water. Starting from the mechanical properties of a shallow formation and based on the basic theory of rock-soil mechanics, the stress distribution around a borehole was analyzed. It was found that the rock or soil on a borehole is in the plastic yield state before the effective tensile stress is generated, and the effective tangential and vertical stresses increase as the drilling fluid density increases; thus, tensile failure will not occur on the borehole wall. Based on the results of stress calculation, two mechanisms and leak-off pressure prediction models for shallow sediments in deepwater drilling were put forward, and the calculated values of these models were compared with the measured value of shallow leak-off pressure in actual drilling. The results show that the MHPS (minimum horizontal principle stress) model and the FIF (fracturing in formation) model can predict the lower and upper limits of leak-off pressure. The PLC (permeable lost circulation) model can comprehensively analyze the factors influencing permeable leakage and provide a theoretical basis for leak-off prevention and plugging in deepwater drilling.

Fracturing Pressure of Shallow Sediment in Deep Water Drilling

The shallow sediment in deep water has weak strength and easily gets into plastic state under stress concentration induced by oil and gas drilling. During drilling, the formation around a wellbore can be divided into elastic zone and plastic zone. The unified strength theory was used after yielding. The radius of the plastic zone and the theoretical solution of the stress distribution in these two zones were derived in undrained condition. The calculation model of excess pore pressure induced by drilling was obtained with the introduction of Henkel’s excess pore pressure theory. Combined with hydraulic fracturing theory, the fracturing mechanism of shallow sediment was analyzed and the theoretical formula of fracturing pressure was given. Furthermore, the influence of the parameters of unified strength theory on fracturing pressure was analyzed. The theoretical calculation results agreed with measured results approximately, which preliminary verifies the reliability of this theory.

Fuzzy Ball Drilling Fluid for CBM in the Ordos Basin of China

Fuzzy ball drilling fluids have been developed in order to effectively control lost circulation during CBM drilling. Depending upon fuzzy balls and colloids in fuzzy balls, the fuzzy ball drilling fluids changed their shapes and properties to completely plug underground heterogeneous seepage channels so as to strengthen the pressure bearing capacity of formations. This paper describes the available features of the fuzzy ball drilling fluid including efficient plugging, good carrying and suspension, formation damage control, compatible weighted by any weighted materials without auxiliary equipment. The fuzzy ball drilling fluids can finish drilling in low pressure natural gas zone, control CBM leakage; control the natural fractures, drilling in different pressures in the same open hole, combination with the air drilling mode, etc. during Ordos CBM drilling. The fuzzy ball drilling fluid will not affect down-hole motors and MWD. The fuzzy ball drilling fluid will be blend simply as conventional water based drilling fluids. The existing CBM drilling equipment can completely meet the fuzzy ball drilling mixing and it is maintained conveniently. The fuzzy ball drilling fluid is the efficient drilling fluid.

Study on Borehole Stability of Unconsolidated Sandstone in Depleted Reservior

Reservoir pressure will decline generally along with production in the oil and gas development process. There are some problems such as borehole collapse or reduced diameter and lost circulation in drilling of initial production stage in unconsolidated sandstone. As the formation pressure declines the stress around borehole changes, and then collapse pressure and fracture pressure are affected. Especially in directional wells, variation of wellbore stability is more complex with different borehole deviation and azimuth. The calculation models of collapse and fracture pressure in depleted reservoirs were established, and relevant data in unconsolidated sand reservoir of an oilfield in Bohai Sea was used to calculate collapse pressure and fracture pressure of directional wells in the condition of pressure depletion before and after. The results showed that collapse and fracture pressure decreased as formation pressure depletion, and safe drilling fluid density window was wider when drilled to the direction of minimum horizontal principle stress. The calculation results can be reference to drilling design of adjustment wells in unconsolidated sandstones.

Physical simulation experiment research on sand control for Lishui gas field

Based on field/local data and cores of the Lishui 36-1 gas field, the formation characteristics and the grain size distribution of the reservoir of the field was analysed specifically. Combined with the empirical approach and the reference of the completion pattern in near fields, the screen types and their size scopes used in this block were determined preliminarily. Then using our sand control physical simulation experiment device for gas well, the analysis of five different screen sizes was completed for Lishui 36-1 gas field. According to the experiment result and the production requirement of this gas field, 140 microns metallic cottons screen is chosen for sand control to meet the local condition. It satisfies the production requirement, and also minimises the sand production amount, which reduces the serious problem of clogging, erosion and wearing the sea pipeline. The evaluation process has been widely used in other offshore gas fields. [Received: June 22, 2012; Accepted: June 29, 2013]

Research on Solutions of the Influences of High Pressure Repeating Packing Sand Control on Productivity

When the well with high-pressure packing sand control becomes invalid a relatively thick sand-gravel mixing zone will exist outside the casing. The mixing zone can hardly be cleared up in the process of later repeating sand control, so with the multiplicity of sand control increasing, the thickness of the mixing zone increases accordingly, which will do harm to the productivity of well. Aiming at this problem, the authors first analyzed the packing zone outside casing and then three optional solutions were given: fracturing packing sand control, casing external gravel packing zone acid pickling, and high-pressure variable diameter gravel packing technique. Especially, the authors proposed the technique of acid pickling of casing external gravel packing zone, and its technical ideas and main points were expounded for the first time; according to the Saucier method and the arch bridge theory, the authors analyzed the optimization of gravel size in the technique of high pressure variable diameter gravel packing sand control, and provided theoretical design principle.