Chengyuan Xu

Lost-Circulation Control for Formation-Damage Prevention in Naturally Fractured Reservoir: Mathematical Model and Experimental Study

Drill-in fluid loss is the most important cause of formation damage during the drill-in process in fractured tight reservoirs. The addition of lost-circulation material (LCM) into drill-in fluid is the most popular technique for loss control. However, traditional LCM selection is mainly performed by use of the trial-and-error method because of the lack of mathematical models. The present work aims at filling this gap by developing a new mathematical model to characterize the performance of drill-in fluid-loss control by use of LCM during the drill-in process of fractured tight reservoirs. Plugging-zone strength and fracture-propagation pressure are the two main factors affecting drill-in fluid-loss control. The developed mathematical model consists of two submodels: the plugging-zone-strength model and the fracture-propagation-pressure model. Explicit formulae are obtained for LCM selection dependent on the proposed model to control drill-in fluid loss and prevent formation damage. Effects of LCMmechanical and geometrical properties on loss-control performance are analyzed for optimal fracture plugging and propagation control. Laboratory tests on loss-control effect by use of different types and concentrations of LCMs are performed. Different combinations of acid-soluble rigid particles, fibers, and elastic particles are tested to generate a synergy effect for drill-in fluidloss control. The derived model is validated by laboratory data and successfully applied to the field case study in Sichuan Basin, China.

Comprehensive prediction of dynamic fracture width for formation damage control in fractured tight gas reservoir

Developed fractures are beneficial for the economic and efficient development of tight gas reservoir. But they will lead to drill-in fluid loss and induce serious formation damage. Fracture width prediction is the key for reasonable selection of particle size distribution to prevent drill-in fluid loss and control formation damage in fractured tight gas reservoir. However, the reservoir fracture width is not constant but changed with effective stress variation induced by drill-in fluid invasion, which make it more difficult for fracture width prediction. In this paper, we develop a comprehensive method to predict the reservoir dynamic fracture width. This method is based on stress-dependent permeability experiment and finite element simulation which are conducted to determine the in-situ fracture width and dynamic fracture width. The in-situ width is used as the initial condition for the simulation. According to the experiment and simulation results for North-Western Sichuan tight gas reservoir, the in-situ fracture width is 3.28-18.59 µm and the dynamic fracture width is 17.89-763 µm. Based on the dynamic fracture width prediction, reasonable particle size distribution can be designed to prevent drill-in loss and control formation damage effectively. [Received: January 27, 2014; Accepted: September 20, 2014]

An Experimental Study on Porosity and Permeability Stress-Sensitive Behavior of Sandstone Under Hydrostatic Compression: Characteristics, Mechanisms and Controlling Factors

Porosity and permeability stress-sensitive behavior of sandstone is investigated through porosity and permeability measurements under hydrostatic compression. An empirical logarithm model is applied to the evaluation of stress sensitivity. Mercury injection, casting thin sections and scanning electron microscope are adopted to discuss the microscopic controlling factors and mechanisms of stress-sensitive behavior of sandstone. Disparities between laboratory-scale and field-scale data are also discussed. Experimental results show that both porosity and permeability decrease with increasing effective stress, and permeability is more sensitive to stress. The evolution of porosity and permeability with increasing effective stress follows the logarithmic model and can be classified into three stages: rapid decline, moderative decline and stable-slight decline stage. Stronger permeability stress sensitivity is observed in specimens with lower initial porosity and permeability, which is fundamentally controlled by the size and shape of void spaces as well as the content and distribution of soft minerals. While porosity stress sensitivity is observed to be discrete. Pore throat with small aspect ratio and irregular shape, fractures and minerals with lower elasticity modulus lead to the increase in permeability stress sensitivity of sandstone. The stress sensitivity characteristics are essentially the result of the dissimilar deformation responses of pores and framework grains under effective stress. Stress sensitivity evaluated in the laboratory usually does not completely represent the stress sensitivity in the field, due to stress release, drilling induced fractures and the difference of experimental and in situ conditions.

Mathematical Model and Experimental Study on Drill-In Fluid Loss Control and Formation Damage Prevention in Fractured Tight Reservoir

Drill-in fluid loss is the most important cause of formation damage during drill-in process in fractured tight reservoirs. Lost circulation material (LCM) addition into drill-in fluid is the most popular technique for loss control. However, traditional LCM selection is mainly performed by trial-and-error method, due to lack of mathematical models. The present work aims at filling this gap, by developing a new mathematical model to characterize the performance of drill-in fluid loss control using LCM during drill-in process of fractured tight reservoirs. Plugging zone strength and fracture propagation pressure are the two main factors affecting drill-in fluid loss control. The developed mathematical model consists of two sub-models, i.e., the plugging zone strength model and the fracture propagation pressure model. Explicit formulae are obtained for LCM selection based on the proposed model, in order to control drill-in fluid loss and prevent formation damage. Laboratory tests on loss control effect by different types and concentrations of LCMs are performed. Plugging pressure and total loss volume are measured and compared with modeling results. Effects of LCM mechanical and geometric properties on loss control performance are analyzed, for optimal fracture plugging and propagation control. Different combinations of acid-soluble rigid particles, fibers and elastic particles are tested in order to generate a synergy effect for drill-in fluid loss control. The derived model is validated by laboratory data.