Abstract

Oil-base drilling fluid density changes considerably with temperature and pressure. Significant density changes prevailing in oil-base drilling fluids can seriously affect the neutron and density sonde readings. The effects of oil-base muds on these logs are discussed, and a technique to account for such effects is proposed. A new approach for true porosity determination in proposed. A new approach for true porosity determination in gas-bearing formations is also presented for cases where the gas is miscible with the invading fluid. Two well examples representing different lithologies, fluid types, and miscibility conditions are presented. These examples confirm the need for the proposed presented. These examples confirm the need for the proposed corrections and their drastic impact on the qualitative and quantitative interpretation of neutron and density logs.

Introduction

Twenty years ago, oil-base muds were used in rare and very specific cases; however, they have now become part of everyday drilling of deep wells. Well log interpretation techniques, mostly developed for water-base mud environment, have to be reviewed and adjusted for the possible impact of a different drilling fluid type.

The examination of log data obtained in several wells drilled with oil-base muds indicated that porosity values derived from the neutron and density logs require environmental corrections to account for fluid properties specific to oil-base muds. Overlooking these effects resulted in qualitative and quantitative misinterpretations; in fact, oil- and water-bearing formations appeared erroneously to be gas bearing.

It was also observed that oil-base muds are used mostly in deep wells. At prevailing high temperatures and pressures, the oil-base fluid may become miscible with the formation gas in the invaded zone. This phenomenon has yet to be incorporated in formation evaluation.

This paper presents a means of accounting for environmental effects on neutron and density tools. It also presents a method of detection and evaluation of gas miscibility effects.

COMPENSATED NEUTRON LOG (CNL)

The compensated neutron tool response is determined mostly by the abundance of hydrogen in the environment investigated. The depth of investigation of the tool typically does not exceed 12 in. into the formation. Because of the shallow radius of investigation, the borehole environment affects to some extent the tool response. Environmental effects are determined by: borehole diameter, mud cake thickness, borehole salinity, mud weight, and borehole temperature and pressure.

The dual detector sonde was designed to reduce environmental effects on the measurements. This tool is calibrated to yield true porosity values in an 8-in. borehole drilled with fresh water at porosity values in an 8-in. borehole drilled with fresh water at 60 degrees F and 14.7 psia, in a formation composed of pure limestone and no mud cake or tool standoff. It is still necessary, however, to apply corrections when borehole conditions deviate considerably from those used to calibrate the sonde. These corrections can be applied using the chart in Fig. 1.

Available literature does not address the corrections required for the CNL in oil-base mud systems. In this study a sensitivity analysis was performed for all parameters present in normal logging conditions. Results show that the effects of borehole salinity, mud weight, and mud cake thickness can be neglected. Results also indicate that in oil-base mud systems corrections for borehole size, temperature, and pressure effects cannot be overlooked.

Using published data, the following expression was derived to correct the neutron porosity effects. The corrected neutron porosity, nc, is expressed by: porosity, nc, is expressed by: [1]

where:

[2]

[3]

[4]

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