Formation damage mechanisms associated with drilling and completion fluids for deepwater reservoirs

The permeability damage of high‐rank coal reservoirs is the main factor affecting the development of coalbed methane (CBM) in the late production stage. Coal core samples from southern Qinshui Basin were saturated with actual formation water, 1/2 salinity formation water, and distilled water to study the swelling of clay minerals. Then, the mechanism of permeability change caused by water sensitivity and its effect on gas production was discussed. The results show that the permeability decrease caused by flow velocity sensitivity did not occur during the water sensitivity experiments. The permeability damage caused by water sensitivity is mainly the static damage from swelling of clay minerals, especially illite/smectite mixed clays and chlorite. During the process of CBM development, the water sensitivity of coal reservoirs will lead to the sharp increase of Na+ concentration and a decrease of Mg2+ concentration in the produced water. Meanwhile, the clay minerals swell due to the adsorption of more Mg2+ and Ca2+, decreasing the reservoir permeability and gas production rate.

Section: Introductionmentioning

confidence: 99%

Swelling of clay minerals and its effect on coal permeability and gas production: A case study of southern Qinshui Basin, China

Tao

Gao²,

Pan

2019

“…Therefore, accurate characterization of types and space for tight oil in pore throats has great significance for creating a scientific evaluation and the effective development of the reservoir. There are three frequently used methods for characterizing the microscopic pore structure in a low permeability, tight reservoir [10][11][12][13] : (a) two-dimensional (2D) image observation, including optical microscopy and field emission scanning electron microscopy (FESEM), which provides qualitative and semiquantitative descriptions of the pore structure according to the 2D images; (b) three-dimensional (3D) volume reconstruction, including micro CT-scans, nano CT-scans, and focused ion beam field emission scanning electron microscopy, which provide 3D descriptions and evaluation of the pore structure and connectivity; (c) quantitative volume evaluation, including low-temperature gas adsorption, highpressure mercury injection (HPMI), rate-controlled mercury injection, and nuclear magnetic resonance (NMR), which provide a quantitative evaluation of the pore structure and reservoir space. 7 During the same period, several Chinese scholars have studied the anisotropy of micro pores, including the distribution of pore throats, pore types and clay matrix, and summarized the research results and methods for determining the pore structure of tight reservoirs.…”

Section: Introductionmentioning

confidence: 99%

Influence of micro‐pore structure in tight sandstone reservoir on the seepage and water‐drive producing mechanism—a case study from Chang 6 reservoir in Huaqing area of Ordos basin

Ren

et al. 2019

To study the influence of pore structure on the seepage and water‐drive producing mechanisms, various methods were combined to describe the micro‐pore structure in the Chang 6 tight sandstone reservoir in the Huaqing area, Ordos Basin, China. Nuclear magnetic resonance (NMR) was combined with displacement experiments to determine the distribution of oil and water in pores of different scales before and after water flooding. There are few micro pores in the reservoir. As permeability increases, the distribution of nano pores decreases, while sub‐micro pores increase. Also, sub‐micro pores are the main pathway within the reservoir. There is a negative power function correlation between the minimum starting pressure gradient of the oil (Swc) and maximum throat radius. Also, with a decrease of permeability, smaller pore throats become more abundant and the nonlinear section of the flow velocity‐differential pressure curves increase. There is a large amount of crude oil gathering in the nano pores. As permeability increases, the main sources of movable oil are from the nano pores (kg < 0.4 × 10−3 μm2), sub‐micro pores (kg ≈ 0.4 × 10−3 μm2‐1.0 × 10−3 μm2), and micro pores (kg > 1.0 × 10−3 μm2). Displacement efficiency is always the highest in sub‐micro pores.

“…The influence factors of shale borehole stability are very complex, which can be attributed to three aspects: physicochemical factors, mechanical factors, and engineering factors . At present, a lot of researches have performed relatively adequate research on physicochemical factors and mechanical factors …”

Section: Introductionmentioning

confidence: 99%

Large‐scale experiments of the borehole instability on shale formation influenced by drill pipe rotation

Sun

Meng

Dai

et al. 2019

Wellbore instability happens mostly in the shale formation. Early researches have studied thoroughly on the physicochemical and mechanical mechanism that causes instability. However, only a few works have considered the influence of field drilling operations, such as drill pipe rotation. In this paper, a true triaxial cell is used to simulate the downhole field situations, and a drill pipe rotation device is innovatively designed and implemented into the true triaxial cell. Using this facility, the influence of drill pipe rotation on shale instability has been performed. Different rotation speed, weight on bit, drilling fluid pressure, and microcracks on borehole have been considered. The enlargement of the wellbore is quantitatively plotted and compared.Results show that the drill pipe rotation contributes to a more enlargement of the wellbore, and a higher rotation speed causes a more severe collapse. The drill string weight on bit also contributes to the enlargement of the wellbore, with a larger weight on bit induces a larger diameter of the collapsed wellbore. Compared with the rotation speed and weight on bit, the influence of drilling fluid density is relatively less significant. However, the influence of microcracks caused by hitting of drilling tools is profound. For a cracked shale formation, the wellbore diameter can be enlarged for more than 40%. Then, the variation of borehole collapse with time and depth under drill pipe rotation influence is analyzed. In all of these four factors, the significant sequence is microcracks > weight on bit > rotation speed > drilling fluid pressure.In all the collapse phenomena, the collapse is more severe in the minimum horizontal stress direction. This is explained by the analysis of the stress distribution in the whole circumferential angle (360°) of the wellbore. Overall, this work fills the gap of the influence of drilling operations on wellbore instability. K E Y W O R D Sborehole enlargement, drill pipe rotation, true triaxial cell, wellbore instability 2896 | SUN et al.