A Model for Filtration of Water-base Mud During Drilling:
determination of Mudcake Parameters

J. T. Dewan and M. E. Chenevert

ABSTRACT

This paper summarizes an effort, ongoing for the last eight years, to develop a predictive theory of mudcake buildup and filtrate invasion practical enough to be utilized in everyday drilling operations. During that time, extensive filtration measurements have been made on about 100 water-base muds, about 2/3 being lab-mixed and 1/3 field muds. With this data base we have arrived at a mathematical model, and developed a corresponding numerical simulator, which allows determination of the necessary mudcake parameters and which quantitatively reproduces virtually every aspect of filtration seen in the laboratory. Extension of the model to predict filtration downhole is planned and will be presented at a later time.

We find that a minimum of three and a maximum of six parameters are required to characterize a mudcake. The basic three are porosity, permeability and a compressibility exponent that describes the dependence of the first two on pressure across the mudcake. If the mudcake exhibits negligible hysteresis when pressure across it is first cycled a single compressibility factor suffices. If it does exhibit hysteresis a second compressibility factor is required. The three (or four) parameters can be determined from a static filtration test, which could be performed at the wellsite. For those mudcakes which erode, two additional parameters, which define the shear strength of the mudcake under zero pressure and with applied pressure, must be determined. This requires a dynamic filtration test, best performed in a regional mud lab.

Examples are shown utilizing muds covering a wide range of compositions and weights. Derivation of mudcake parameters is illustrated and comparison between simulated and measured filtration sequences is presented.

A Computer Algorithm for Obtaining Rw from the SP Log

W. W. Souder

ABSTRACT

Most engineers and geoscientists participating in petrophysics training programs dislike the chart book technique for deriving Rw from the spontaneous potential log. This process can be done on a computer or calculator quite easily if the chart for converting Rw to Rwe (and the inverse conversion Rwe to Rw) is represented mathematically. Equations are presented in this paper that describe Rw as function of Rwe and temperature (and the inverse transformation Rwe as a function of Rw and temperature) which very closely match the curves in the Schlumberger chart SP-2. These equations can easily be utilized in a computer to calculate Rw from the static spontaneous potential, temperature, and Rmf values for a formation. Sample functions written in Visual Basic are provided and can be used in an Excel spreadsheet or as an independent program on a Windows PC.

Impact of Dynamic Mud Invasion on Sw Estimation in Fresh Water Formations

Sudan Blocks 1/2/4, Muglad Basin

Guan Yingchun

ABSTRACT

Greater Nile Petroleum Operating Company’s (GNPOC) contract area Blocks 1/2/4 is located in the Muglad Basin, Southern Sudan, a continental rift basin developed during late Jurassic to early Cretaceous time. The main oil-bearing reservoirs are the Cretaceous Bentiu, Aradeiba, Zarqa and Ghazal formations, sandstones deposited in fluvial environments ranging from braided streams, meandering channels, to fluvial deltaic settings.

The presence of extremely fresh formation water (salinity 1000–4000 ppm) in these formations coupled with the use of saline drilling mud (salinity 20,000 ppm) results in significant invasion effects. This problem could be further aggravated with prolonged formation open-hole exposure time during the drilling operation.

The dynamic mud invasion effect poses a unique petrophysical challenge in GNPOC’s contract area whereby the measured resistivity response is not representing the true formation resistivity (Rt) but rather an apparent resistivity (Rta) which is always lower than Rt. As a result, potential oil-bearing zones could be missed and Sw calculation based on Rta could be over-estimated, consequently under-estimating reserves. Inconclusive petrophysical evaluation of fluid types caused by mud filtrate invasion often results in numerous production tests, sometimes up to 5 tests per well, a costly decision in formation evaluation.

A more accurate Sw can be estimated by using simulation software on the dynamic response of resistivity. This extrapolates the measured resistivity value backward in time to the Rt value present before mud invasion effects develop. Alternatively, the use of logging while drilling (LWD) could provide Rt before mud invasion occurs. LWD coupled with intermediate logging runs in the target formation in key exploration wells would provide a reliable calibration set for the simulation software to obtain more reliable Rt values from wells with conventional logs. It is estimated that evaluation of Sw using this approach could increase the present GNPOC’s booked reserves by about 20%.

It is recommended that all existing GNPOC wells be re-evaluated using the above approach to correct for dynamic mud invasion effects and to screen for any missed pay zones for updating of reserve estimates in GNPOC’s year 2000 Annual Review of Petroleum Resources (ARPR).

Induction Logging with Directional Coil Polarizations: Modeling and Resolution Analysis

Arvidas Cheryauka, Michael Zhdanov, and Motoyuki Sato

ABSTRACT

We present a novel method of formation conductivity estimation based on an induction logging tool with multicomponent transmitters and receivers. We describe a modeling study of the tool prototype with tilted transmitting and receiving coils. Theoretically, this induction system can be considered as a triaxial multisensor array of controlled polarization. The sensitivity analysis and induction tool response simulations are based on dyadic Green’s function calculations in the layered media and the integral equation method. We also consider the spatial resolution of induction logging with tilted coils and its ability to delineate formation properties and geometry.