Minimising filtrate loss into the formation by forming a filter cake with low porosity and permeability near the wellbore region is the key to managing formation damage problem. Visualising and understanding the structure of a filter cake can help formulate a drilling fluid system that provide an effective caking process with reduced water loss. In this study, a technique that identifies the appropriate mud caking process is suggested. It can be applicable to relevant sandstone formations within 2 inches from the wellbore. This identification technique is based on the measurement of filtrate loss, the measurement of the return permeability of cores exposed to selected muds and the visual observation on the micro-level of cake structures of these muds in order to describe them and highlight their merits. A good mud system is the one which have a low filtrate loss, a low return permeability and a filter cake structure close to that of the ideal internal and external filter cake. This study also provides an investigative and comparative look at the internal and external filter cake structures of different muds using the Scanning Electron Microscopy. This technique verified that the glycol and petrofree drilling fluids possess the characteristics and filtration properties of good mud systems. The glycol filter cake structure was superior to those of the solid free /low solid KCl-polymer muds. Scanning Electron Microscopy provided clear and descriptive images of the structures of internal and external filter cakes. Introduction Over the years, numerous published papers investigated the negative effects of drilling fluids on prospective formations, especially the region adjacent to the Wellbore (Jiao and Sharma, 1992; Marx and Rahman, 1984, 1991; Longeron et al, 1995, Krueger, 1986). Although it serves many important purposes, the introduction of drilling fluids into the virgin formation during the drilling process results in the majority of cases in an irreversible permeability impairment leading to low productivity. For safety reasons, most of the wells are drilled overbalanced, and this differential pressure between the wellbore and across a permeable formation leads to filtration of the drilling mud. Solid particles and mud filtrate are lost to the formation, first at a high rate and then at a decreasing rate due to the deposition of the mud solids on the wall of the borehole (filter cake formation process) to a constant value once the mud solids deposition rate is equal to that of the solids re-entrainment in the mud flow (equilibrium condition). P. 203^
The matrix reactivity of sandstone formations with mixtures of hydrofluoric (HF) and hydrochloric (HC1) acids has been studied experimentally using natural cores. A systematic approach, which includes laboratory analysis and computer modelling, has been used to design and plan acid treatment for sandstone formations. Matrix reactivity to acid mixtures (reaction rate) and the relationship between the porosity and permeability are established by subjecting the Pacoota Sandstone core samples to different acid concentrations and injection rates at different temperatures. Based on material balance and reaction kinetics a numerical simulator has been developed and verified in the laboratory. This simulator can adequately predict spent-acid concentration and changes in porosity and permeability as a function of acid penetration depth for given acid treatment conditions (acid concentrations, injection rates and treatment temperatures).
Wells drilled with nitrified drilling fluids require a solution for the transmission of measurement-while-drilling (MWD) surveys, bi-directional communication with rotary steerable systems, and transmission of MWD and and logging-while-drilling (LWD) measurements of downhole temperature and annular pressure for surface choke adjustments. Wired drillpipe (WDP) provides the solution for these requirements. Results from a recent well drilled into an underpressured reservoir in southern Mexico provided an opportunity to demonstrate the applicability of WDP to deliver the required measurements and maintain the proper directional control while keeping the well fluids under control. During WDP operations, both the traditional mud pulse transmission and the new WDP transmission methods are available, providing 100% reliability for receiving the downhole MWD/LWD data.During the drilling of this well, both single and multi-phase Managed Pressure Drilling (MPD) techniques were used. The plan called for the overburden and reservoir sections to be drilled in one bit run but with different mud types and constant bottomhole pressure (BHP). Before drilling the reservoir section, the single-phase mud used to drill the overburden had to be changedover to multi-phase mud while monitoring the wellbore for signs of instability. Maintaining constant BHP in this type of MPD operation is complicated by the fact that current hydraulic models do not have the proven capability to support constant BHP in a nitrified OBM.With mud pulse telemetry, downhole data transmission stops when the rig pumps are shut-down. But with WDP, downhole data is actively transmitted during the time between pump shut-down and pipe disconnection. This allows MPD personnel to monitor actual annular pressure during pump transitions and more accurately determine the optimum choke position for constant BHP. During the connection, the downhole annular pressure is stored in memory. Once the connection has been made the data is transmited up-hole for evaluation and analysis, which provides immediate feedback on the stability of the BHP during the connection.Pressure sensors (along string measurements or ASM) within the multiple WDP repeater subs allow us to compute the fluid density at multiple intervals along the annulus for the first time in the history of drilling. We document a rather surprising case of temperature effects overriding pressure effects during the use of a single-phase compressible OBM used in the first stage of this drilling operation. This has implications for using WDP measurements to calibrate and verify hydraulic models for both single and multi-phase drilling fluids. Proper hydraulic modeling capabilities are critical for MPD operations.This well provides the opportunity to demonstrate other applications for these newly invented "interval fluid densities." For example, they are used to verify the top of the fluid level with and without the presence of nitrogen injection when accurate flowin versus flow-out measurements are not available. This is ...
Wellbore instability and formation damage are the two major problems encountered by the petroleum industry. It is commonly accepted that formation damage is mainly caused by fluid-rock interaction due to the change in pore fluid chemistry which is caused by invading mud filtrate. Invasion of mud filtrate can be reduced by forming a tight filter cake on the wellbore wall. A tight filter cake can also provide support to the wellbore wall and prevent wellbore collapse. Therefore, the most effective option for solving wellbore instability and formation damage problems is to design a drilling mud that is compatible with formations in relation to both fluid-rock interaction and mud caking characteristics. This paper considers a number of mud systems with novel features and investigates their potential use in drilling and completion of tight gas formations in Central Australia, which are highly susceptible to formation damage. Among the four (4) muds investigated, ester based mud has been found to be the most effective in reducing formation damage by producing a tight filter cake on the wellbore wall. P. 113
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