The Influence of the Borehole EnvironmentUpon Compressional Sonic Logs
Mark Alberty: BP Exploration, Houston, Texas

Abstract: The observed compressional sonic recorded with conventional sonic devices is affected by formation and/or filtrate water salinity, dissolved gas, borehole and formation temperature, pore pressure, low saturation biogenic free gas, and the presence of hydrocarbons. The magnitude of this effect is not well appreciated by users of borehole sonic logs. These factors can have a significant effect upon measured interval transit times and our ability to interpret sonic logs for porosity or upon ties to seismic velocities or amplitudes. Examples presented demonstrate that these factors can affect interval transit times by 10 or more microseconds in unconsolidated sandstones.

Conventional sonic equations such as the Wyllie time average or the Raymer-Hunt travel time equation provide little insight to the influences these environmental factors may have upon conventional sonic logs. Results of experimental characterization of the influence of these factors upon water compressibility is used to characterize the influence of these environmental factors upon sandstone reservoirs using the Gassmann relationship. The presented results provide an appreciation of the sensitivity of these factors upon measured (Dt) values. The understanding provides a mechanism to correct sonic logs for this influence, which leads to more accurate interpretation of sonic logs for porosity and fluid content and improved ties to seismic and checkshot velocities.

Downhole Lateral Nuclear Magnetic Sounding
S. M. Akselrod, V. I. Danevich, D. M. Sadikhov, and V. A. Mamed-zadeh, Main Department of Geophysics of Azerbaijan State Oil Company

Abstract: Determination of nuclear-magnetic properties of rocks at different distances from the borehole [i.e., lateral nuclear magnetic sounding (LNMS)] provides the possibility of enhancing formation evaluation. The LNMS can be performed by a special nuclear magnetic tool operating either in the Earth’s magnetic field or in the field of permanent magnets.

One of the most important problems that can be solved by means of LNMS is determining the character of the flushed-zone radial inhomogeneity, which is, in turn, a function of the saturating fluid and the reservoir’s porosity and permeability.

Downhole experimental measurements performed using a LNMS tool operating in the Earth’s magnetic field proved the possibility of delineating mud-contaminated zones and of distinguishing between gas-bearing and oil- or water-bearing reservoirs. Quantitative determination of the free fluid index (Iff) of shallow and deeper parts of the flushed zone makes determination of the reservoir’s porosity and permeability more accurate.

Gravity Methods: Useful Techniques for Reservoir Surveillance
J. L. Brady and D. S. Wolcott: ARCO Alaska, Inc., C. L. V. Aiken: University of Texas at Dallas

Abstract: Understanding the movement of gas in oil reservoirs is important because premature breakthroughs or high gas-oil ratios significantly increase costs and can reduce ultimate recovery. The Prudhoe Bay and Kuparuk River oil fields, located on the North Slope of Alaska, have an expanding gas cap and a gas storage area, respectively. Significant gas movement in both reservoirs has occurred, since discovery, and the vertical and areal extent are not as well defined as desired. The current methods of monitoring are expensive and increasingly less reliable because of well conditions.

In a search for better methods to monitor gas movement, borehole and surface gravity techniques were identified. The Borehole Gravity Meter (BHGM), because of its large radius of investigation, can potentially predict the true gas-oil contact, irrespective of a localized gas cone and other near-wellbore completion problems. Bypassed lenses of oil and intervals of gas under running oil can be identified. The data from the BHGM can provide the vertical distribution of gas and oil at a well. The new elevator sonde for the borehole gravity meter may significantly improve the vertical resolution of the tool in deep deviated wells. To better understand the areal distribution of gas, high-resolution surface gravity data can be used. As the gas cap propagates outward and/or downward, a negative gravity anomaly is created as a result of the displacement of oil by gas.

The results show that significant gravity anomalies have occurred in the Prudhoe Bay and Kuparuk River oil fields as a result of gas replacing oil. The gravity anomalies can be measured with borehole gravity and in certain situations with surface gravity instruments. New surveying techniques are needed to obtain increased vertical and lateral resolution for reservoirs buried as deep as the North Slope fields. The use of global positioning systems (GPS) is an economically viable method for obtaining accurate surface positions for locating surface gravity stations. Surface gravity data resolution can be improved further by stripping and downward continuation.

Additional developments to improve deviation and size limitations of the BHGM, data acquisition and processing of surface gravity, and testing of GPS in Arctic latitudes are required. Plans to field test these techniques and provide data to design new techniques and tools are ongoing.

A Laboratory Procedure for Estimating Irreducible Water Saturation From Cuttings
Rosemary Knight Dept Geophysics and Astronomy and Dept of Geological Sciences, University of British Columbia, Paulette Tercier Dept of Geophysics and Astronomy, University of British Columbia, David Goertz Dept of Geophysics and Astronomy, University of British Columbia

Abstract:At low levels of water saturation, the water in the pore space of a rock becomes hydraulically disconnected and effectively immobile. This level of water saturation is referred to as the irreducible water saturation Swi. The magnitude of Swi is an important consideration in formation evaluation and is determined by laboratory measurements on core samples or plugs using porous plate displacement or centrifuge techniques. We have investigated the use of an alternate laboratory technique that uses a measure of drying rates of samples and works with samples of any size—from full cores to small chips and cuttings.

As a fully water-saturated sample dries, the drying rate is initially constant and is controlled by the rate of evaporation at the sample’s outer surfaces. During this constant rate period, the water in the sample has a high degree of hydraulic connectivity and is drawn through capillary transport to the sample surface. As the level of water saturation decreases, the water loses hydraulic connectivity and the drying process enters the falling rate period. The transition from the constant rate period to the falling rate period has been associated with the onset of irreducible saturation in a sample. Because of sample-scale saturation heterogeneity, this volume-averaged Swi is considered to be an upper limit for the true Swi for the sample.

We have conducted a series of laboratory experiments in which we measured the drying rates of sandstone samples as a function of saturation level. We used two sandstones, Berea sandstone and a tight gas sandstone. For each sandstone we obtained drying rate data on a range of sample sizes from 1-in. (2.5-cm) plugs to small chips the size of cuttings. Each sample was fully saturated with water and then left to dry on a weigh scale interfaced to a computer to record weight as a function of time; weight was converted to water saturation. In the data for each sample we clearly see the transition from the constant rate period to the falling rate period and select the saturation level that marks the end of the constant rate period as the estimate of Swi for that sample. Because a decrease in sample size results in reduced saturation heterogeneity, we see a corresponding decrease in the estimated Swi and conclude that samples the size of cuttings yield the most accurate measure of Swi for the sample. Using the drying rate data, we obtain for Berea sandstone Swi=0.14; for the tight gas sandstone we obtain Swi=0.49; both values agree well with previous laboratory measurements.

We conclude that the collection of drying rate data is a rapid and simple laboratory procedure that could be used with cuttings to obtain a good estimate of Swi.