The Principle of a Critical Invasion Rate and Its Implications for Log Interpretation

Abstract

The interpretation of MWD and wireline logs of invaded formations can be enhanced by an understanding of the controls on invasion rates. Experimental investigation of dynamic filtration rates has shown that under certain conditions these are dependent only on the hydraulic shear stress at the mudcake surface for a given mud composition. They are virtually independent of differential pressure and independent of porous medium permeability (as opposed to mudcake properties) over porous medium permeability (as opposed to mudcake properties) over a 1:10 range. For very low permeability media however, the clear fluid Darcy flux may already be below the dynamic filtration rate. Then no mudcake forms and filtration rates obey Darcy's law. Comparison with theory suggests that there is a critical flux (filtration rate per unit area) above which day particles accrete irreversibly to the mudcake; below this flux they cannot stick at all. Invasion volumes at the time of wireline logging are usually dominated by fluid loss that occurred under conditions of dynamic filtration. The conjecture of a critical dynamic filtration rate, dependent only on mud composition and shear, means that invasion volumes should be independent of mud overbalance pressure and of formation properties (permeability, saturation, far field pressure, internal mudcake) unless formation permeability is so low that it limits the filtration flux to less permeability is so low that it limits the filtration flux to less than the critical flux. Then, formation properties control fluid loss. The permeability at which the behavior changes depends on both mud overbalance pressure and mud flow rate. These results are important for the interpretation of MWD and wireline logs in the presence of invasion. Field examples of multiple pass MWD and wireline resistivity logs and invasion volume computations from a single pass of wireline resistivity and porosity logs often indicate trends in the invasion volume within a zone or between zones. These trends may be interpreted as indicating differential formation resistance to invasion at low permeabilities but must be due to factors other than the loss rate at moderate to high permeabilities. One such factor is vertical movement of the filtrate under the Influence of gravity. Introduction Log analysts often seen to infer reservoir dynamic properties by interpreting the invasion profile at the time of wireline logging, or changes in the invasion profile between MWD and wireline logging times. An understanding of the influences on the rates at which filtrate is lost to the formation from the wellbore, particularly for those conditions under which the largest net volumes are lost, is a first step in this process. Fluid loss is conventionally divided into three categories: beneath-the-bit, static filtration and dynamic filtration. Fluid loss beneath the bit occurs as fresh rock is exposed. The loss rate is highest herb but ends within seconds, as pores near the wellbore are bridged, forming an internal mudcake. From published estimates one may conclude that the total invasion depths from volume lost in this phase are unlikely to much exceed half a wellbore radius. it is thus not of first importance for evaluating fluid loss at timescales exceeding half an hour. After the bit passage, further fluid loss involves mud filtration forming an external mudcake with concomitant invasion of the formation by mud filtrate. It is usual to distinguish two regimes: static and dynamic. These are really misnomers, since they refer only to mud flow in the well, not to the magnitude or time development of the loss rate. Dynamic filtration (referred to in chemical engineering as crossflow filtration) occurs when mud is flowing past the surface of the mudcake; static filtration takes over when the mud flow is stopped. For static filtration, the loss rate q(t) declines indefinitely as mudcake grows, in principle until the mudcake entirely plugs the wellbore. In dynamic filtration, q(t) declines until the mudcake stops growing and q(t) reaches (asymptotically) a steady state, with a nearly constant loss rate qD. Usually the majority of the fluid volume lost during time intervals of interest in formation evaluation is lost under dynamic conditions. This is because for most reservoir rocks, loss rates are greater for dynamic conditions than for static and because mud is circulating for the majority of the time for safety and hole deaning purposes. purposes. We describe in this paper simple semi-quantitative models for a complex process, concentrating on the principal features understood from existing literature and research, discounting effects and influences of secondary importance. Simple formulas are given; these deliberately sacrifice any attempt at high accuracy to serve as straightforward conceptual models for the major features of wellbore filtration. Most of what follows refers to orthodox water based muds, with bentonite as the primary viscosifier and fluid loss control material. More experimental data are required for other mud types to be confident in the applicability of these ideas to them. FLUID LOSS RATES IN DYNAMIC AND STATIC FILTRATION How the flow rate per unit area q(t) changes over time is significantly different between static and dynamic conditions for large times t. For small times however it turns out that the two regimes are closely similar. For this reason we consider static filtration first. P. 39

Sampling While Drilling: An Emerging Technology

“…The general performance and utility of this class of spectrometer is wellknown in the industry, and the lessons garnered from its use have led to the development of more-capable spectrometers (Fujisawa et al 2008). In the SWD tool, the spectrometer's primary use is fluid identification and the estimation of contamination in real time.…”

Understanding the Impact of the Dynamic Environment When Acquiring Formation Pressures Whilst Drilling

“…While a well is drilled and mud circulation is ongoing, the dynamic filtration rate (Fordham et al 1991) reaches an asymptote (steady state). However, changes to the mud circulation rate such as during rig connections, and mechanical scraping from BHA stabilizers with drill pipe rotation will disturb the already formed mudcake, altering the filtrate rate and hence the supercharging in the near wellbore.…”

Characterization of Sampling-While-Drilling Operations

“…Second, decreasing and then increasing the mud-circulation rate also appears to have little or no effect in this particular experiment. It is known from the work of Fordham et al (1991) that given sufficient time, an equilibrium-filtration rate determined by the mud-circulation rate is achieved under dynamic-filtration conditions. Again, there appears to be little effect on the contamination of the pumped fluid of changing the mud-circulation rate, as determined by the observed trend in optical density.…”

“…A rough estimate of the time scale required to reach steady filtration under dynamic conditions yields a time of 30 minutes(Fordham et al 1991).…”