Explicit Pore-Pressure Evaluation: Concept and Application
- Jean-Louis Alixant (Louisiana State U.) | Robert Desbrandes (Louisiana State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- September 1991
- Document Type
- Journal Paper
- 182 - 188
- 1991. Society of Petroleum Engineers
- 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.12.1 Measurement While Drilling, 5.3.4 Integration of geomechanics in models, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.1 Hydrates, 3 Production and Well Operations, 5.2 Reservoir Fluid Dynamics, 3.3.1 Production Logging, 1.7.5 Well Control, 1.12.2 Logging While Drilling, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 4.3.4 Scale, 1.6 Drilling Operations, 1.6.6 Directional Drilling, 5.6.1 Open hole/cased hole log analysis
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A general method was developed to determine pore pressure in undercompacted shales on the basis of explicit relationships among measured shale properties, shale porosity, effective vertical stress, and pore pressure. Although calibration may be necessary, this technique neither pressure. Although calibration may be necessary, this technique neither uses empirical correlations nor requires establishment of normal trend lines. A modular approach allows users to implement custom equations. This paper outlines the concept will focuses on its application to real-time pore-pressure evaluation based on resistivity measurements.
The main difficulty in pore-pressure evaluation during drilling is that direct pressure measurement is impossible. Thus, overpressured formations must be characterized with a measurable pressure-dependent parameter that can be used to infer pore pressure. pressure-dependent parameter that can be used to infer pore pressure. Earth scientists agree that compaction disequilibrium of argillaceous sediments accounts for the vast majority of overpressures encountered in tertiary sedimentary basins. This observation changes the problem of overpressures into one of undercompacted shales, which are characterized by typical porosity trends.
Conventional Methods. Shale porosity is not directly accessible during drilling, but it affects such measurable parameters as resistivity and acoustic travel time. Hottmann and Johnson extracted pore-pressure information from these parameters with empirical pore-pressure information from these parameters with empirical correlations. That "conventional" approach was extended to the interpretation of drilling-rate measurements, refined, and generalized. Conventional interpretation techniques are based on the determination of a "normal trend" that represents the variations of the monitored shale-porosity-dependent parameter with depth in the hydrostatic interval. Departures from the normal trend are interpreted qualitatively in terms of pressure-regime variations and quantified by means of an empirical correlation that relates the normalized porosity-sensitive parameter directly to the pore-pressure gradient. parameter directly to the pore-pressure gradient. Two main limitations generally are associated with this implicit approach. 1. The determination of the normal trend is a subjective task that may be troublesome without regional experience. 2. An empirical correlation between petrophysical measurements and fluid-pressure gradients must be established on the basis of a regional data set. Despite these shortcomings, use of conventional pore-pressure evaluation methods based on resistivity or acoustic travel-time wire-line logs or rate-of-penetration (ROP) measurements is standard practice in the drilling industry. Eaton's correlations, in particular, have proved to be fairly reliable and are still used.
Modern Needs. The drilling industry has experienced several important changes during the last decade. The advent of measurement while-drilling (MWD) tools and polycrystalline-diamond-compacbits, the increased diversity of drilling environments, and the rapid development of horizontal drilling technology have necessitated a reassessment of the pore-pressure evaluation concept. pore-pressure evaluation concept. One main requirement of modem drilling technology is real-time capability. To determine the normal trend line within a reasonable degree of confidence, the operator must ensure that sufficient real-time data are available for the hydrostatic interval. To obtain these data, an MWD tool must be run in the upper section of the wellbore, which increases the cost and the risk of tool failure. When hydrostatic data are being acquired, defining the normal trend during drilling is challenging without regional experience, as the following example illustrates. Fig. 1 is a plot of the shale-resistivity data available at a given time during drilling. At that time, the normal trend line can be positioned as shown, which places the top of the overpressured shale interval at about 7,600 ft [2300 m]. Additional data become available as drilling progresses, revealing that the early interpretation is erroneous (Fig. 2). In fact, the top of the overpressured interval appears to be at 10,400 ft [3200 m]. That mistake could have caused an unnecessary mud weight increase or premature setting of the casing string. premature setting of the casing string. This example suggests that a real-time pore-pressure evaluation method should not use normal trend lines; i.e., to maximize the benefit of a real-time data-acquisition system, interpretation of the measurements in terms of pore pressure should be possible as they are made. Conventional pore-pressure evaluation methods do not make optimal use of MWD technology, and a new approach is required.
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