This paper describes the results of a field application of borehole gravimetry to measure secondary gas saturations in a fractured limestone reservoir. Owing to its deep-reading capability and insensitivity to near-wellbore effects, the borehole gravimeter succeeded where conventional cased-hole logging methods had failed. Borehole-fluid pressure data, recorded together with the gravity data, proved useful in ensuring that the density data had the necessary high accuracy. This paper additionally presents modelling results that indicate the potential usefulness of time-lapse borehole gravity data for monitoring flood fronts remote from a borehole. This application would benefit from the development of a sensor with a very reliable absolute calibration and low drift. Introduction The potential of borehole gravimetry In hydrocarbon exploration and production was recognised already in 1950 by Smith, and the results of such measurements have been reported since 1966. Several applications of the technique have become established, and additional applications have been suggested on the basis of modelling studies (see, for example, Ref. 4). Borehole gravimetry has attractive characteristics. A borehole gravimeter (BHGM) has a large radius of investigation, and the formation bulk density derived from its gravity data is hardly influenced by the borehole fluid, casing and near-borehole features, such as mud-filtrate invasion and drilling-induced formation disturbance. Therefore, the BHGM tool is suitable for use in a cased hole. In fact, it provides the only method of obtaining the important formation bulk density measurement through casing. Its deep density measurement can sometimes be turned to good advantage in the evaluation of exploration or appraisal wells. The benefits of BHGM density measurements could be even larger if the tool were used more widely in the area of reservoir management. The tool could be applied, for example, to monitor fluid saturations averaged over a large volume or the position of flood fronts remote from a borehole. Modelling studies of these two applications have been reported, but to our knowledge no field applications have been documented. P. 151^

Time-lapse seismic has shown many successful offshore applications, but has turned out to be much more cumbersome when applied onshore. Successful applications are mainly observed for shallow objectives and large acoustic impedance changes, such as thermal EOR and CO2 injection. The dominant problems for onshore time-lapse are caused by near-surface variations between base and monitor surveys [Pevzner 2011]. By taking appropriate measures in acquisition and processing it is possible to overcome these problems. We demonstrate that with a continuous seismic time-lapse field trial in Schoonebeek, The Netherlands, where thermal EOR is applied. The time-lapse measurements not only enable us to observe pressure and temperature variations in the reservoir, but also to quantify these variations. These measurements complement the measurements made in wells, enabling the reservoir engineer to construct more accurate dynamic models, which make it possible to make better reservoir management decisions. We will describe the steps taken in acquisition and processing of the data and show an interpretation of the measurements.

TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractBeginning in 2003 Petroleum Development Oman (PDO) began testing the crosswell electromagnetic method for waterflood monitoring in Oman. The tomographic method, which determines interwell resistivity from inductive EM signals, is well suited for tracking injected water volumes, especially when this fluid is a low resistivity, high salinity brine.The method was applied in two Omani oil fields for tracking injected water. In the first field several vertical well pattern pilots were established to measure water flood sweep efficiency in two zones of a shallow, but relatively thick, multi layer reservoir. In the shallow zone a two year monitoring program revealed that much of the injected water had bypassed the reservoir, likely escaping through high permeability streaks. In the deeper pilot the EM results showed an excellent interwell sweep but some leakage to the overlying formation. For the second field the method was applied in existing widely-spaced barefoot injection wells. In this old line drive flood we wished to image the residual oil saturation and injected water volumes. In this case the resistivity anomalies in this mature water flood were fairly subtle. Interwell formation heterogeneity could only be revealed by calculating resistivity differences between the initial or background model and the image after inversion. One of these difference images revealed that a significant oil bank remained adjacent to the central producer.The overall experience with the crosswell EM tomography revealed that this technique can have great value in imaging water flooding. Drawbacks include the costs of installing special monitoring wells that are required under some conditions and the low resolution that is obtained in cases where the length of the survey interval is short relative to the well spacing. A good reservoir model is required in order to use constraints that are required for steering the crosswell EM inversion.

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