In this paper, we present a new approach for modeling filtrate invasion during the drilling of a horizontal well with fractures, and the impact of its cleanup on well performance. The approach incorporates the drilling schedule and the experiment-based dynamic filter-loss data into multiphase reservoir simulator. Unlike the traditional leak off model in which the assumption of the piston-like displacement in the filtrate invaded zone, the fluid flow in the invaded and the reservoir zones are described by use of more realistic two-phase oil-gas flow equations which are solved under the dynamic boundary conditions of the leak off model and time-varying reservoir exposure due to drilling, tripping, completions and workovers. Since, impact of fractures on both invasion and flow-back is more pronounced in tight formations, this paper will focus on such formations.
In real drilling process, the initial dynamic mud-cake formation is critical for controlling filtrate loss. A dynamic fluid-loss model which reflects the spurt loss, non-Darcy and non-Newtonian characteristics of filtrate flow through the mud cake is coupled with the reservoir simulator. Mud properties and different events influence cake compression, dynamic cake deposition and cake erosion as well. The application of the dynamic filtrate loss model avoids the complexity in building a multi-parameter mathematical mud cake model without loss of generality. As in previous works, the dynamic filtrate loss model is based on actual special core tests.
Sensitivity analysis is conducted to study the influence of dynamic leakoff coefficient on the saturation distributions during the filtrate invasion and flowback. In existing experiments, only the leakoff coefficients for matrix are measured. The extrapolation of the dynamic leakoff coefficients for simulation of fluid loss into intersecting fractures is discussed. Driven by Buckley-Leverett equations, theoretical analysis is presented to emphasize the quantitatively spatial correlation between the invaded filtrate saturation and spatial permeability reduction in the invaded zone. Moreover, well performance influenced by the water blocking, relative permeability alteration and the damaged permeability variation are simulated. A horizontal well example is used to illustrate the flexibility of this approach. The results show that the dynamic leakoff coefficient significantly impacts the clean-up and well performance. The results also shed light on the relative value of hydraulically fracturing a conventional over-balanced drilled horizontal well, versus hydraulically fracturing the well after it is drilled underbalanced.
Introduction
Overbalanced drilling and workover operations invariably cause invasion of the drilling and workover fluids. Although the drilling fluid loss can be reduced by use of additives, the higher overbalance and longer exposure time of reservoir to drilling fluids may still result in formation damage.
Fluid loss in rotary drilling is a dynamic process and can be divided into three phases. In the first phase, during the initiation of the mud cake, spurt loss dominates filtrate invasion, i.e. the drilling fluid is lost before a mud cake is established. In the second phase, the mud cake is growing and filtrate loss rate is considered to be proportional to the square root of time. In the third phase, the mud cake properties tend to be stable under the joint influences of hydrodynamic forces, differential filtration pressure and shear stress. In practice, assessment of formation damage can be conducted either using laboratory-based approach at core scale1–6 or using numerical modeling technology at field scale7–11. Civan1 has published an excellent literature survey regarding the existing approaches for the evolution of formation damage. Here, we briefly review of the numerical approaches for simulation of formation damage caused by filtrate invasion.
