© 2020, WAJES All Rights Reserved 22 World Academics Journal of ___________________________________________ Research Paper. Engineering Sciences Vol.7, Issue.4, pp.22-27, December (2020) E-ISSN: 2348-635X Modeling the Rate of Mud Invasion in a Permeable Formation Kingdom Kponanyie Dune 1 , Bright Bariakpoa Kinate 2* , Adaobi Stephenie Nwosi-Anele 3 1,2,3 Department of Petroleum Engineering, Rivers State University, Nigeria * Corresponding Author: baa2rex@yahoo.com Available online at: www.isroset.org Received: 29/Sept/2020, Accepted: 05/Dec/2020, Online: 31/Dec/2020 Abstract—The invasion of mud filtrates in to the permeable formation is an inevitable experience during drilling operations. It is usually associated with near-complete formation damage in the immediate vicinity of wellbore and the response of well log tools (especially the low penetration tools) are then greatly influenced by the mud filtrates instead of formation fluids which makes its interpretation more difficult. Hence, it is often necessary to know the possible spatial and time variation of mud filtrate concentrations in the porous media during invasion. This study presents a model technique for the numerical simulation of rate of mud invasion using a pressure transient analogy of mud concentration gradient. The model uses a finite difference element of 1000ft 3 divided into five (5) uniform gridblocks of length, 2ft each. The results of the study showed clearly that from the time and space of invasion process initialisation, the mud concentration diffuses into the porous medium. At early time of invasion, the mud filtrate is continuously diffused into the porous media while at later stage of invasion, mud concentration builds from the inner gridblocks facilitating the formation of filter cake at the wellbore. Further analysis of the results reveals that the displacement of the in-situ formation fluid by the invading mud is not a piston-like phenomenon as shown by the spatial variation of the mud filtrates concentration during the invasion period. However, the concentration gradient can be used to define the extent of mud filtrate invasion into the permeable formation Keywords—Mud Concentration, Mud Flow rate, Mud invasion,Numerical Simulation, Permeable Formation I. INTRODUCTION During overbalanced drilling, a positive pressure differential exists between the mud hydrostatic pressure and the formation pressure. As a result, the adjacent formation is inevitably invaded by mud filtrates during mud filter cake build up. Basically, three types of mud filtration has been identified to be associated with drilling operations- beneath the bit filtration, static filtration and dynamic filtration[1,2].These filtration processes consequently result to three distinct zones of mud invasion into the permeable formation namely: an invaded zone extending to some depth into the formation, the internal filter cake of about few inches into the formation and the external filter cake deposited on the walls of the borehole [3]. The presence of mud filtrates in the immediate vicinity of the wellbore remarkably influences the response of well log tools and its measurements [4]. Since well log tools can only penetrate few inches into the formation, it is almost certain that the depth of invasion may have extended relatively farther. Hence, the logging tool at this point is significantly influenced by the mud filtrate properties instead of the in-situ fluid.This is particularly true for shallow investigation tools such as CNL (Compensated Neutron log), LDT (Litho Density log), MLL (Micro Later log) etc[5]. The implication of this is that there can be misinterpretations in the reservoir rock and fluid properties. Over the years, the idea of mud invasion has made the interpretation of shallow logging tools very critical to interpret. In some cases, deep penetration tools are rather resorted to as a result of its ability to penetrate beyond the invasion zones. However, these tools still do not see clearly enough and therefore still have remarkable uncertainties in the measurements which are relatively compensated for by introducing correction factor terms[6]. The method of interpretation has been based on the assumption that the mud filtrate invasion front advances in a radial piston-like fluid saturation front [7]. However, [8] showed that the piston-like frontal advancement does not hold for low permeability formations.This is also supported by [9] whose study on mud filtrate depth of invasion at the wellbore vicinity revealed that displacement of in-situ fluid by the mud does not conform to the piston- like assumption. Several factors responsible for mud filtrate invasion have been investigated by several works to include dynamic filtration rate; rock petrophysics, fluid properties and capillary pressure; bed geometry; mud properties, cake parameters and time{ [10][11][12][13][ 14]}. Modeling the rate of the invasion become necessary after identifying the rate as one of the factor responsible for invasion to understand and minimize formation filtrate invasion. The Rest of the paper is organized as follows, the next Section contains related work and context of study area, Section III contain details of the modelling and essential data used for the work, Section IV describe the results and their discussion and, Section V concludes the research.