Detecting high overpressure GLENN L. L. B OWERS , Applied Mechanics Technologies, Houston, Texas, U.S.
Normal pressure is pore fluid pressure that equals the
hydrostatic pressure of a column of formation water extending to the surface. Overpressure Overpressure is pore fluid pressure greater than normal pressure. However, However, no standard definition exists for what constitutes high overpressure. What can be said is that high overpressure often means trouble. For an explorationist, it could mean blown reservoir seals; for a driller, it could mean excessive time spent fighting formation fluid influxes and/or drilling fluid losses. A practical upper limit for pore pressure is the overburoverburden stress. Pore pressures in this range are on the verge of opening fractures that can vent fluid and bleed off pressure like a pressure relief valve. Therefore, criteria for defining high overpressure are sometimes expressed in terms of a percentage of the overburden stress, say, pore pressure greater than 90% of the overburden stress. In this article, high overpressure will be defined simply as pore pressure that approaches the overburden stress. All but one potential cause of overpressure can produce high pressure. Fortunately, Fortunately, the mechanism that cannot generate high pressure is the most common cause of overpressure. Therefore, detecting high overpressure basically boils down to determining where extraordinary overpressure mechanisms may be encountered. Overpressure detection detecti on is based on the premise that pore pressure affects compaction-dependent geophysical properties such as density, resistivity, and sonic velocity. Shales are the preferred lithology for pore pressure interpretation because because they are more responsiv responsivee to overpres overpressure sure than most rock types. Consequently, overpressure detection centers around shale deformation behavior. Shale deformation behavior. For stress ranges of practical interest, shale compaction is controlled by the difference between between total applied stress stress and pore fluid pressure. pressure. This difference, termed the effective stress, stress, represents the portion of the total stress carried by the rock grains. Figure 1 illustrates the effective stress concept with laboratory data for Cotton Valley shale (Tosaya, 1982). In tectonically relaxed environments, compaction can be related to the vertical effective stress. The nature of this relationship depends on a formation’s stress history; specifically specificall y, on whether the vertical effective stress has ever been higher than it currently is. Nondecreasing effective stress states. Clayey sediments deposited on the seafloor can have porosities in excess of 80% and sonic velocities near the speed of sound in water. Under increasing effective stress, the sediments compact, and their density, resistivity, and sonic velocity asymptotically approach limits set by the properties of the sediment grains. The effective stress re relation lation followed by a compaction-depencompaction- dependent geophysical property for nondecreasing effective stress states is referred to as its virgin curve. curve. Effective stress reductions. Compaction is predominately an inelastic process. Therefore, only a small amount of elastic rebound occurs when the effective stress acting on a formation is reduced (unloading (unloading). ). Instead of following the virgin curve, rebound occurs along a flatter effective stress path. During reloading, the rebound curve is retraced until the past maximum effective stress is reached, and inelastic deformation resumes. Figure 2 compares laboratory reloading data for Cotton Valley shale (Tosaya, 1982) with estimates of 174
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Figure 1. Laboratory example of the effective stress principle (from Tosaya, 1982).
Figure 2. Shale compaction/rebound behavior.
Figure 3. Pore structure models used to characterize shale behavior. Pores with aspect ratios in the 0.001-0.1 range undergo most rebound. Higher aspect ratio pores are too rigid. Crack-like pores are too flexible (Bowers, 2001).
the original virgin curves. Bulk properties versus transport properties.Sonic properties. Sonic velocity and resistivity generally undergo more elastic rebound than bulk density and porosity (Bowers and Katsube, 2002). This is the case with the Cotton Valley shale data in Figure 2. The common threads are that porosity and density are bulk properties, ties, while sonic velocity and resistivity are transport properties. properties.
Figure 4. Response of vertical effective stress to different overpressure mechanisms.
Figure 6. Wireline overpressure indicators. Shale resistivity, sonic velocity, and density data fall below their normal trends. Resistivity changes not related to pore pressure can be caused by variations in pore water temperature and salinity.
Figure 7. Detecting reversals without high overpressure. Vertical projections of temperature-corrected resistivity, sonic velocity, velocity, and density from a point inside the reversal intersect the logs at a common point above the reversal. Figure 5. Overpressure caused by lateral transfer. Isolated sands A and B follow the the shale far-field pore pressure curve (thick red line). Dipping sand C-D transfers deep shale pressures updip.
Bulk properties only depend on net pore volume, while transport properties are sensitive to pore sizes, shapes, and how pores are interconnected. Bowers and Katsube propose that a rock’s pore space consists of a combination of relatively large, high aspect ratio storage pores linked together by a network of lower aspect pores, with transport properties controlled by ratio connecting pores, the connecting pores (Figure 3). Storage pores undergo primarily inelastic volume losses, while the more flexible connecting pores are capable of elastic rebound. Consequently, Consequently, effective stress reductions cause connecting pores to elastically widen without significantly changing storage pore sizes. As connecting pores widen, they increase flow path sizes available for conducting electrical current, and decrease the number of intergranular contacts for transmitting sound. The net effect apparently has a much larger impact on transport properties than bulk properties—suggesting that an indicator of in-situ rebound (unloading) is a depth interval in which sonic velocity and resistivity data appear anomalously low in comparison to bulk density measurements. Overpressure causes. The causes of overpressure can be divided into four general categories: undercompaction, undercompaction, fluid expansion, lateral transfer, and tectonic loading. Undercompaction cannot produce high overpressure. The other three mechanisms can. The conditions that produce normal pressure and the four types of overpressure are
described below. Normal pressure. Normally pressured formations are able to maintain hydraulic communication with the surface during burial. Consequently, their pore fluid can easily be squeezed out to accommodate compaction, and their pore pressure follows the hydrostatic pressure curve for formation water. The upper portion of Figure 4 characterizes normal pressure conditions. Effective stresses in normally pressured environments continually increase with depth. Undercompaction. Overpressure most commonly occurs when low permeability prevents pore fluid from escaping as rapidly as pore space tries to compact. Excess pressure builds as the weight weight of newly deposited deposited sediments sediments squeezes squeezes the trapped fluid, a process referred to as undercompaction or compaction disequilibrium (Figure 4). Undercompaction typically occurs where there is a transition from a sand-prone to a shale-prone environment. For an impermeable seal and an incompressible pore fluid, pore pressure would increase at the same rate as the overburden stress once sealing occurred. Less perfect seals and more compressible pore fluids would reduce overpressure. The key point is that undercompaction can never drive pore pressure toward the overburden stress curve. This also means that undercompaction cannot cause effective stress reductions. Fluid expansion. Overpressure can be generated within the pore space by flui by fluidd expansi expansion on mechanisms mechanisms such as heating, hydrocarbon maturation, and the expulsion/expansion of intergranular water during clay diagenesis (Bowers, 1995). Here, overpressure results from the rock matrix constraining the pore fluid as the fluid tries to increase in volume. Load transfer from smectite grains to pore water during illitization is another potential way clay diagenesis can cause F EBRUARY EBRUARY 2002
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overpressure (Lahann, 2002). In very low permeability shales, fluid expansion can produce high overpressure (Gordon and Flemings, 1998), particularly when acting in tandem with undercompaction (Figure 4). Unlike undercompaction, fluid expansion can cause the effective stress to decrease (unloading) as burial continues. continues. Most people associate unloading with overburden losses caused by erosion. Fluid expansion reduces reduces the load carried by a rock’s grains in the same way that pumping fluid into a hydraulic jack reduces reduces the load carried by a car’s springs. Lateral transfer. Fluid expansion-like overpressure can also result from a sealed interval having pore fluid pumped into it from another, higher-pressure zone. Sometimes this can be caused by charging along faults. It can also occur along a dipping sand enclosed in shale. As illustrated in Figure 5, the sand transmits pore fluid and pore pressure from deeper shales updip, a process known as lateral transfer (Yardley and Swarbick, 2000). Lateral transfer can generate crestal pore pressures high enough to fracture overlying shale seals, especially when there are long gas column. Tectonic loading. Trapped pore fluid squeezed by tectonically driven lateral stresses induces overpressure overpressure in the same way that undercompaction does. However, unlike undercompaction, tectonic loading is capable of generating generating high overpressure (Yassir (Yassir and Addis, 2002). This also means that tectonic loading can cause vertical effective stress to decrease, but in tectonic environments, compaction is no longer controlled by vertical effective stress alone.
Figure 8. High-pressure well example. Sonic and resistivity logs undergo reversals not seen by the density log. Pore pressures are underestimated when undercompaction is assumed the cause of overpressure (Equivalent Depth Solution). Curve R are raw resistivity data; curve R200 are resistivity data corrected to a common temperature of 200°F (93°C).
Figure 9. Detecting reversals rever sals with high overpressure. overpress ure. Vertical Vertical projections of temperature-corrected resistivity, resistivity, sonic velocity, and density from a point inside the reversal do not intersect the logs at a common point above the reversal; the density log is crossed at a deeper depth.
Detecting pressure conditions. During burial under normal pressure conditions, the effective stress continually increases with depth. Density, resistivity, and sonic velocity proceed up their respective effective stress virgin curves. Because each effective stress can be mapped to a particular depth, density, resistivity, and sonic velocity data can be replotted versus depth. The depth profile that a compaction-dependent geophysical property would follow during burial under normal pressure conditions is termed its normal trend. trend. Overpressure Overpressure prevents the effective stress from increasi ncreasing as rapidly as it would during burial under normal pressure conditions. Consequently, the onset of overpressure (“top of overpressure”) generally occurs where a compactiondependent geophysical property first departs from its normal trend (Figure 6). In the deepwater Gulf of Mexico, overpressure can start within a few hundred feet of the seafloor. Undercompaction cannot cause elastic rebound; it simply slows the rate at which compaction proceeds along the virgin curve. Therefore, the compaction state of a formation overpressured by undercompaction can never be less than it was at some earlier point in time. On depth plots, classic signs of undercompaction are density, resistivity, and sonic velocity data that (1) continue increasing or (2) remain con176
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stant after they depart from their normal trends (Figure 6). Undercompaction can also be the source of overpressure overpressure when density, resistivity, and sonic velocity logs go through reversals, reversals, meaning, they all drop below values at some shallower depth (Figure 7). It is tempting to use the depth trend followed by a wireline log as an indicator of the burial history followed by each point along the log. If this were true, then the only way a reversal could occur is through elastic rebound. And that would mean undercompaction could not be the only source source of overpre overpressur ssure. e. However, However, it is also possible that point C in Figure 7 simply stopped compacting once it became sealed at point A. In general, if undercompaction is the source of overpressure, overpressure, any two depths with identical compaction-dependent properties and similar lithologies should have identical vertical effective stresses (the Equivalent Depth Method for pore pressure estimation is based on this concept). Therefore, each point within a reversal should have at least one corresponding point above the reversal with the same density, density, temperature-corrected resistivity (Traugott, 1997), and sonic velocity values. This defines a simple way to determine whether undercompaction is the cause of overpressure overpressure when density, resistivity, and sonic velocity go through reversals. Pick a point at the same depth in each reversal, and project each point vertically until it crosses its log above the reversal. If all three
logs are crossed at similar depths, overpressure was caused by undercom undercompaction paction.. When fluid expansion causes cau ses high overpressure, it causes unloading, and therefore elastic rebound. So, the search for high overpressure is basically a search for rebound. One indicator of elastic rebound is a reversal, but not all reversals are caused by rebound. Therefore, we incorporate the observation that bulk properties undergo less rebound than transport properties. This means that sonic velocity and resistivity data in highly overpressure intervals should undergo larger reversals than bulk density measurements. Figure 8 shows an example of a high pressure well. The fourth track compares pore pressure measurements (RFTs) (RFTs) and mud weights used to drill the well with pore pressures pressures computed from the sonic velocities using the Equivalent Depth Method. The sonic velocity and resistivity logs undergo reversals not seen by the density log, and Equivalent Depth Method underestimates pore pressure. In cases in which density, resistivity, and sonic velocity all go through reversals, the approach used to identify undercompaction can also be used to detect high overpressure. Pick a point at the same depth in each reversal, and project each point vertically until it crosses its log above the reversal. If the density log is crossed at a deeper depth than the sonic and resistivity logs, that is an indicator of high overpressure (Figure 9). Because shales are not 100% impermeable, lateral transfer should have a detectable effect on shales in the vicinity of the sand. However, it is not unusual for shale geophysical data to show no evidence of lateral transfer. Why this happens remains an open question. The bottom line is that it does. Therefore, seismic evidence of sands with significant signifi cant vertical structure should raise two red flags: (1) pore pressures near the crest of the sand may be high and (2) shalederived pore pressures may not reflect sand pressures. Because tectonic loading creates overpressure by squeezing the rock, it will not make the rock less compacted. In fact, it could make the rock more compacted. Consequently, Consequently, tectonically induced overpressure generally cannot be detected from either bulk density or porosity measurements (Yassir and Addis, 2002). The response of resistivity and sonic velocity is less clear. These properties can be direction-sensitive, so it seems possible they could detect a drop in the vertical effective stress. However, it appears (Hottman et al., 1979; Yassir and Bell, 1996; Hennig et al., 2002) that this typically is not the case. These masking effects are further exacerbated when tectonic loading pushes formations closer to the surface. Most accumulated compacti compaction on remains locked in, so uplifted overpressured formations may be more compacted than normally pressured rocks. Overall, geophysical measurements tend not to be reliable indicators of tectonically induced overpressure.
Suggested reading. “Pore-pressure estimation from velocity data; accounting for overpressure mechanisms besides undercompaction” by Bowers (SPE Drilling and Completions, 1995). “Pore/fracture pressure determinations in deep water” by Traugott (Deepwater Technology, supplement to World Oil, 1997). “Generation of overpressure and compaction-driven fluid flow in a Plio-Pleistocene growth-faulted basin, Eugene Island 330, Offshore Louisiana” by Gordon and Flemings ( Basin Research, 1998). All 2002 references in this article can be found in: Pressure Regimes in Sedimentary Basins and Their Prediction, edited by Huffman and Bowers, AAPG Memoir 76, 2002. “Lateral transfer: a source of additional overpressure?” by Yardley and Swarbick ( Marine and Petroleum Geology , 2000). Acoustical Properties of Clay Bearing Rocks by Tosaya (PhD dissertation, Stanford University, 1982). “Abnormally high fluid pressures and associated porosities and stress regimes in sedimentary basins” basins” by Yassir and Bell ( SPE Formation Evaluation, 1996). “Determining an appropriate pore-pressure estimation strategy” by Bowers Bowers (OTC 130 13042, 42, 2001 Offshore Offshore Technology echnology Conferen Conference). ce). E L
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Summary. High overpressure affects shale in a fundamentally different way than undercompaction, because it can cause elastic rebound. Therefore, rebound is an indicator of high overpressure. The geophysical signature of rebound is a depth interval in which shale sonic velocity and resistivity data undergo larger reversals than bulk density measurements. High pressure may not be detectable when overpressure results from lateral transfer, and generally will not be detectable with tectonically induced overpressure. However, seismic evidence of either sands with extensive vertical strucstruc ture or significant tectonic activity should itself heighten awareness of the potential for high overpressure. overpressure. F EBRUARY EBRUARY 2002
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