Fluid Contamination Transient Analysis

Abstract

Successful in-situ fluid cleanup and sampling operations are commonly driven by a fast and reliable analysis of pressure, rate, and fluid contamination measurements. Techniques such as pressure transient analysis (PTA) provide important information to quantify reservoir complexity, while fluid contamination measurements are commonly overlooked for reservoir description purposes. We introduce a new interpretation technique to relate fluid contamination measurements with near-wellbore fluid-transport properties by identifying early- and late-time flow regimes in fluid contamination and its derivative function. The derivative methods used in PTA inspired the development of the new fluid contamination interpretation method. Contamination transient analysis (CTA) evaluates transient measurements acquired during cleanup of mud-filtrate invasion to infer important reservoir flow conditions. Center-point derivative methods are applied to the fluid pumpout volume and time evolution of fluid contamination to identify flow regimes in cases of water-based mud invading either water- or hydrocarbon-bearing formations. We document synthetic examples of the new interpretation method for seven reservoir cases, numerically simulated to obtain contamination data, namely, homogeneous isotropic reservoir, radial boundaries, vertical boundaries, thin-laminated formations, mud-filtrate invasion radius, petrophysical properties, and permeability anisotropy. Single-phase flow and multiphase flow cases are also compared in the analysis. Reservoir boundaries and features are identified in the flow regimes obtained from the combined interpretation of the fluid contamination derivative (FCD) and the log-log plot of the contamination transform. The seven reservoir cases assume fixed reservoir and operational parameters, such as reservoir geometry, rock properties, fluid properties, invasion radius, time of invasion, maximum pumpout rate, and maximum drawdown pressure, to allow for a controlled sensitivity analysis enabling the identification of cleanup trends. It is emphasized that real-time field conditions could trigger certain limitations of the transient techniques developed in this work, such as noisy downhole formation testing measurements, active mud-filtrate invasion, or tool failure. To validate the assumptions, observations, and results of the numerical simulations, a field case is examined to (a) highlight the value of CTA in real-time fluid sampling operations and (b) further investigate its limitations. An alternative validation of the method is performed by applying the derivative directly to the formation testing measurements during fluid cleanup, reducing the uncertainty in the contamination estimation and the interpretation of transient trends. The new approach of the FCD is an alternative to improve fluid cleanup efficiency and to detect the spatial complexity of the reservoir during real-time downhole fluid sampling. Using log-log plots of fluid contamination and the FCD method, we encounter characteristic slopes defining late-time flow regimes. The spherical flow regime gives rise to a slope of –2/3, which has been previously documented by homogeneous isotropic analytical models. Radial flow exhibits a steeper slope of –3 that can be detected when the vertical limits are attained. Boundary effects are evident when the late-time slope of the FCD is equal to –1/3. In addition to the detection of reservoir boundaries, the CTA techniques developed in this paper enable the identification of reservoir fluid type and shale laminations and provide a foundation for the quantification of invasion radius and permeability anisotropy. It is found that cleanup efficiency could be improved based on contamination transient analysis by identifying the flow regimes taking place in the reservoir during filtrate cleanup, hence improving the prediction of the time required to acquire non-contaminated fluid samples.