In deep oil and gas drilling, high-temperature and high-salinity conditions have posed severe challenges to water-based drilling fluids. These harsh environments can impair the performance of drilling fluids, increase fluid loss, and lead to frequent downhole accidents. In view of the above problems, this study combines the advantages of zwitterionic and hydrophobic association to prepare a zwitterionic hydrophobic association polymer (P-DADL) with high temperature and salt resistance. The chemical structure and properties were characterized and evaluated by in-situ infrared spectroscopy, nuclear magnetic resonance, thermogravimetry, SEM and other experimental methods. The results show that the thermal decomposition temperature of P-DADL is greater than 297 ℃, and it has a spatial network structure in aqueous solution, which exhibits strong clay-particle adsorption and suspension stability. After aging at 180 ℃ for 16 h, the apparent viscosity of the drilling fluid containing 2 wt% P-DADL is 62 mPa·s, the viscosity retention rate is 70.86%, and the FL_API is only 7.4 mL. Under the conditions of 180 ℃ and 35 wt% NaCl, the FL_API was further reduced to 6.4 mL, reflecting the salt synergistic filtration reduction characteristics. In addition, the suspension stability of saturated brine drilling fluid is greater than 98% within 48 h. P-DADL can effectively enhance the temperature resistance and salt resistance of water-based drilling fluid. Compared with commercial products, P-DADL exhibited a 78.4% improvement in filtration performance relative to commercial products. These findings highlight the potential of P-DADL as a multifunctional additive for high-salinity, high-temperature drilling operations.

During the production, the fluid in the vicinity of the directional well enters the wellbore with different rates, leading to non-uniform flux distribution along the directional well. However, in all existing studies, it is oversimplified to a uniform flux distribution, which can result in inaccurate results for field applications. Therefore, this paper proposes a semi-analytical model of a directional well based on the assumption of non-uniform flux distribution. Specifically, the direction well is discretized into a carefully chosen series of linear sources, such that the complex well trajectory can be captured and the non-uniform flux distribution along the wellbore can be considered to model the three-dimensional flow behavior. By using the finite difference method, we can obtain the numerical solutions of the transient flow within the wellbore. With the aid of Green's function method, we can obtain the analytical solutions of the transient flow from the matrix to the wellbore. The complete flow behavior of a directional well is perfectly represented by coupling the above two types of transient flow. Subsequently, on the basis of the proposed model, we conduct a comprehensive analysis of the pressure transient behavior of a directional well. The computation results show that the flux variation along the direction well has a significant effect on pressure responses. In addition, the directional well in an infinite reservoir may exhibit the following flow regimes: wellbore afterflow, transition flow, inclined radial flow, elliptical flow, horizontal linear flow, and horizontal radial flow. The horizontal linear flow can be observed only if the formation thickness is much smaller than the well length. Furthermore, a dip region that appears on the pressure derivative curve indicates the three-dimensional flow behavior near the wellbore.

The effect of wellbore pressure drop on horizontal well pressure response is relatively important when flow velocity is high or the surface of horizontal wellbore is rough. The objective of this study is to develop a stable and robust algorithm in Laplace domain to analyze horizontal-well pressure with pressure drop along the wellbore. Based on the novel definitions of horizontal well permeability and conductivity, the equation of fluid flow along a horizontal wellbore with pressure drop has the same form as that of fluid flow in a varying-conductivity fracture. A new dimension transformation has been used to change the varying-conductivity model into a constant-conductivity model, and then an iterative procedure has been introduced to obtain the pressure. This algorithm is developed in Laplace domain and eliminates the need for computations in the time domain. Besides, the skin effect and wellbore storage is easily to be taken into consideration.

Natural gas hydrate is an ice-like substance which is sometimes called"combustible ice"since it can literally be lighted on fire and burned as fuel.Natural gas hydrate is characterized by widespread distribution, large reserves and little pollution.This paper introduced the distributions of hydrate, hydrate reserves and properties of hydrate.The main exploration methods, such as geophysical exploration and geochemical exploration have been presented.In addition, the main production techniques of natural gas hydrate including depressurization, thermal stimulation and chemical injection have been summed up.Finally, the challenges and outlooks of natural gas hydrate production have been proposed.