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Summary Several recently published studies discuss the concept of inductive resistivity-logging devices with oblique transmitting and/or receiving coils. Both wireline induction and logging-while-drilling (LWD) propagation resistivity-tool concepts have been considered. Directional resistivity measurements and improved anisotropy measurements are among the benefits promised by this type of device. Analyses based on point-magnetic dipole antennas were used to illustrate these potential benefits. The effects of a metallic mandrel, borehole, and invasion were not considered because of the absence of a suitable forward model. This paper characterizes mandrel, borehole, and invasion effects for a variety of candidate tilt-coil devices with antenna array parameters similar to those of the previous studies. The characterization is based on calculations from a new forward model that includes tilted transmitting and receiving coils of finite diameter embedded in a concentric cylindrical structure. Important details of the forward model used in the calculations are also provided. Introduction Conventional propagation resistivity devices are routinely used for geosteering applications. Because data from these devices have essentially no azimuthal sensitivity, the LWD engineer is greatly aided by a priori information regarding the proximity of the target bed relative to other geologic features such as shales and water-bearing zones. Suitable a priori information is often available from offset logs. In cases in which offset logs are not fully useful because of changing depositional environments or different tectonic settings, azimuthally sensitive resistivity data would improve the quality of the geosteering effort. One way to achieve azimuthal sensitivity to benefit geo-steering (and to use it for imaging) is to construct a tool similar to a conventional propagation resistivity device, but with the transmitters and/or receivers tilted with respect to the axis of the drill collar. In fact, directional resistivity tools(DRTs) have been proposed in the literature for this pur-pose.1–3 To the knowledge of the authors, DRTs have only been analyzed with point-dipolemodels, which ignore both the drill collar and the finite size of the antennas. For apparent lack of a suitable forward model, mandrel, borehole, and invasion effects have not been considered in the literature. A model has been developed that accounts for tilted transmitters and receivers embedded in arbitrary layers of a concentric cylindrical structure. Many important details of this model are discussed in Appendix A. The term mandrel effect is used here to denote the difference between values calculated with a point-dipole model and the model that accounts for the mandrel encompassed by the antennas. Mandrel effects on DRT measurements will be grouped into three categories:Absolute effects where the mandrel primarily attenuates the signals because of a reduction in the magnetic moment of the antennas.Residual effects that remain after an air-hang calibration is applied to the data.Perturbations to the azimuthal sensitivity of the measurement caused by the finite size of the antennas and the drill collar. Algorithms that transform raw tool measurements to resistivity values can be based on computationally simple point-dipole solutions without significantly degrading the accuracy of the results if mandrel effects associated with categories 1 and 2 can be suppressed. For conventional LWD propagation resistivity measurements, mandrel effects of type 1 are addressed by air-hang calibration. Algorithms that suppress type 2 mandrel effects are discussed in the literature.4 Type 3 mandrel effects are not discussed here.

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This paper presents our perspective of the shallow-water flow (SWF) problemin the Deepwater Gulf of Mexico (GOM). The nature of the problem, includingareal extent and overpressuring mechanisms, is discussed. Methods for sandprediction and shallow sediment and flow characterization are reviewed. Theseinclude seismic techniques, the use of geotechnical wells, regional trends, various MWD methods, and cements and settable spots. Finally, examples of flowincidents with pertinent drilling issues, including well failures andabandonment, are described. Total trouble costs due to shallow-water flow for all GOM operators probablyruns into the several hundred million dollars. Though the problem remains aconcern, advances in our knowledge and understanding make it a problem that ismanageable and not the "show stopper" once feared. Introduction SWF may occur while drilling shallow over-pressured formations at deepwatersites. It is a high profile problem in the GOM, though it does occur elsewhere(Ref. 1) and will likely be encountered in other deepwater regions (Fig. 1). Drilling shallow over-pressured sands may cause large and long lastinguncontrolled flows, well damage and foundation failure, formation compaction, damaged casing, and re-entry and control problems. Most spectacularly, eruptions from over-pressured sands may result in seafloor craters, mounds andcracks (Figs. 2 and 3). Eaton2, 3 has described the significant problems causedby SWF in the Ursa area. A recent inspection of 106 wells (Ref. 4) indicatedthat $175 MM has been spent on SWF and prevention and remediation on thosewells. Total industry costs due to SWF likely exceed several $100 MM. The problem is compounded by the difficulty in seismically imaging thesesands (Refs. 5 and 6). This stems from the relatively low sand/shale contrastin acoustic impedance. The impact of this problem on well site selection andwell design is significant, and makes drilling in SWF areas particularlychallenging.

The deepwater Gulf of Mexico is now a major area of activity for the U.S. oil industry. Compaction causes particular concern because most prospective deepwater reservoirs are highly geo-pressured and many have limited aquifer support; water injection may also be problematic. Thus, economic development may require significant drawdowns with ensuing compaction and its attendant problems.To address some of the issues associated with compaction, we initiated a special core-analysis program to study compaction effects on turbidite sand porosity and permeability specifically. This program also addressed a number of subsidiary but no less important issues, such as sample characterization and qual ity, sample preparation, and test procedures. These issues are particularly pertinent, because Gulf of Mexico turbidites are generall y unconsolidated, loose sands, and are thus susceptible to a whole array of potentially serious core-disturbing processes.One key result of the special core analysis program is that turbidite compressibilities exhibit large variations in both magnitude and stress dependence. These variations correlate with creep response in the laboratory measurements. The data suggest creep may be important because observed creep relaxation rates, although slow on a laboratory time scale, are much faster than typical reservoir drawdown rates.The effects of compaction on permeability are significant. To eliminate complicating effects caused by fines movement, we made oil flow measurements at initial water saturation. The measurements indicate compaction reduces permeability four to five times more than porosity on a relative basis.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAs part of the effort to understand the problem of shallow water flow (SWF) in the Gulf of Mexico (GOM), and to obtain data and develop design criteria for offshore production structures, geotechnical wells were drilled in several different prospects. These prospects cover a 200+ mile swath across the central deepwater GOM where SWF control problems have been experienced. In these wells, pore pressure measurements, as well as core and other in-situ measurements, were taken in the deepwater shallow sediments.Key facts and observations from these measurements include the following:1. A majority of the pore pressure measurements were made in low permeability clay rich material, predominant in the shallow sediments. Some measurements were proximal to more permeable sand and silt zones. A few measurements were made in sandy/silty intervals.2. Where significant over-pressures were present they were found to begin at or very near the mudline, and to increase more-or-less linearly with depth below mudline (BML).3. Over-pressures in the shallow sediments of the central deepwater GOM are due primarily to rapid sedimentation rates generated by the Mississippi River depocenter. This is supported by the fact that measured over-pressures exhibit regional trends generally consistent with sedimentation rate. 4. Regional trends in over-pressures are also correlative with drilling experience and the incidence and severity of shallow water flow (SWF) occurrences.5. The degree of over-pressuring is consistent with sediment porosity based on core measurements, i.e., higher over-pressures are associated with higher porosities.6. There are exceptions to these general regional trends. Most notable is that over-pressures in shallow permeable sand or silt zones may not be in equilibrium with their bounding shales. Both pressure depletion and hyper-pressuring have been observed in the geotechnical-well pore pressure measurements and in drilling operations. Thus well location (up dip, down dip), faulting, fracturing and other factors need to be considered in predicting pore pressures in these permeable zones.

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