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Well Engineering the hydrostatic head caused by water is less than that caused by any rock. The resultant effect is that as the water depth increases, the numerical value of the overburden gradient and in turn the fracture gradient reduce. Hence, offshore wells will have lower overburden gradient near the surface due to the influence of seawater and air gap and the uncompacted sediments. In onshore wells, the near surface overburden gradient is influenced mainly by the uncompacted surface sediments. Example 1.3 Overburden Gradient Calculations For Offshore Wells Determine the overburden gradient at various depths for the following offshore well Figure 1.1 Construction of Overburden Gradient 20 18 16 14 12 10 8 6 4 2 0 1.92.02.12.22.32.42.52.6 20 1 8 1 6 1 4 1 2 1 0 8 6 4 2 0 0.700.750.800.850.900.951.00 a. Bulk density g/cc Depth 1000 ft Lower limit of all data points Upper limit of all data point b. Overburden stress gradient psi/ft 20 18 16 14 12 10 8 6 4 2 0 1.92.02.12.22.32.42.52.6 20 1 8 1 6 1 4 1 2 1 0 8 6 4 2 0 0.700.750.800.850.900.951.00 a. Bulk density g/cc Depth 1000 ft Lower limit of all data points Upper limit of all data point b. Overburden stress gradient psi/ft TOCPreviousNextTOCPreviousNext PORE PRESSURE Effects of Water Depth On Overburden Gradient 8 Well Engineering and Thermodynamic Effects TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Depositional Effects Well Engineering the solid layers are squeezed closer together and the pore water is expelled to the sea. The clay sediment has high permeability and porosity 60-90. In this initial state, as long as the rate of sedimentation remains fairly slow, the pore fluid will continue to escape as compaction increases and therefore the clay will continue to exhibit a normal pore pressure see Figure 1.2 If the equilibrium between compaction and expulsion of water is disrupted such that the pore fluid cannot escape, abnormal pore pressure will result. This disruption can result from an increase in the rate of sedimentation reduction in the rate of fluid expulsion caused by i a decrease in permeability due to solids blocking the passages or ii the deposition of a permeability barrier such as limestone or evaporite stringers. When disruption to the normal compaction process occurs, three things happen 1. the same volume of pore fluid remains in the rock 2. porosity of the rock is maintained at the new depth and fluids can not escape and compaction of the rock is prevented TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Depositional Effects Well Engineering its thickness varies from a few feet to thousands of feet. Halite is totally impermeable to fluids and behaves like a sponge highly viscous absorbing the overburden stress from above and then rting equal stresses in all three directions. This is an extremely important property as it indicates that the horizontal stress is equal to the overburden stress. As we shall see in Chapter 5, this stress system requires casing set across salt section to have a very high collapse strength. When salt is deposited, the pore fluids in the underlying ations cannot escape and therefore become trapped and abnormally pressured. Salt Diaparism Diapirism is the piercement of a ation by a plastic, mobile, less dense underlying ation, typically salt. Salt exhibits plastic behaviour at elevated temperatures and pressures and due to its low density will move upwards to salt domes in overlying ations. Salt has no porosity and no permeability and therefore can be a perfect seal.Indeed, the Rotliegendes gas reservoirs in the Southern North Sea owe their existence to the perfect seal provided by the Zechstein salt. TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Depositional Effects Well Engineering clay minerals, methane and carbon dioxide. Thus ations that contain large amounts of volcanic ash may become overpressured due to the production of these gases. Overpressure encountered in the South China Sea and NW Coastal USA is attributed to this cause. 4.1.4TECTONIC EFFECTS Tectonic activity can result in the development of abnormal pore pressure as a result of a variety of mechanisms including folding, faulting, uplift and salt diaparism.Salt diaparism was discussed earlier. TOCPreviousNextTOCPreviousNext PORE PRESSURE Depositional Effects 20 Well Engineering Revolutions Per Minute RPM; bit type; bit wear; hydraulic efficiency; degree of overbalance; drilling fluid properties, hydrostatic pressure and hole size. The difference between the mud hydrostatic pressure and pore pressure is called the overbalance or “Chip Hold Down Pressure CHDP“. This overbalance prevents ation fluids from entering the wellbore while drilling. However, this overbalance CHDP also acts to keep the rock cuttings held to the bottom of the wellbore. The effects of bit rotation and hydraulics offset this force and ensure that cuttings are lifted from the bottom of the hole.The CHDP differential force has one of the largest effect on ROP especially in soft to medium strength ations. If all parameters affecting ROP are held constant whilst drilling a uni shale sequence then ROP should decrease with depth. This is due to the natural increased compaction with depth reflecting a decrease in porosity and increased shale density and increased shale compressive strength. When entering an abnormally pressured shale, the drillbit sees a shale section which is undercompacted. The increased porosity and decreased density of the undercompacted section results in the ation becoming more ‘drillable’ as there is less rock matrix to remove. Consequently ROP increases, assuming all drilling parameters were kept constant. In addition, the reduced differential pressure less CHDP between the mud hydrostatic and pore pressure further increases ROP. The increase in ROP on entering an abnormally pressured zone is shown in Figure 1.7. As can be seen from this figure a sharp drillbit would pick up the onset of the transition zone much faster than a dull bit. Careful monitoring of ROP during drilling is useful in detecting the onset of an abnormal pressure, however, the value of the ROP parameter on its own is limited as outlined below. TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Corrected D Exponent Well Engineering b the drill string being in contact with the low side of the hole on a deviated well and c due to ineffective hole cleaning. Rotating torque often increases in an abnormally pressured zone due to the physical encroachment of the ation most notably shale into the borehole. In addition, as pore pressure increases and differential pressure decreases, the increased porosity and fluid content of the ation, combined with a decrease in chip hold-down pressure, allow greater penetration of the rock by the drillbit. This leads to an increase in ROP and a greater friction increased torque between the bit and the rock. However, increasing torque is also caused by drilling deviated holes, out of gauge holes and bearing wear. PPPn dcn dco ------- - PP9 1.5 1.1 ------ - TOCPreviousNextTOCPreviousNext PORE PRESSURE Gas Levels 34 Well Engineering knocking off the ation by the drill string on trips; ineffective hole cleaning and poor suspension properties of the drilling fluid. Variations in drag, torque and fill are usually encountered in both normally pressured and abnormally pressured zones. Hence these indicators should be used in conjunction with other indicators to give a definitive indication of drilling into an abnormally pressured zone. The same apply to any of the indicators that are discussed in this book. The author recommends the use of at least four indicators, preferably plotted on the same plot. 6.4GAS LEVELS Hydrocarbon gases enter the mud system from various sources during drilling operations. The gases are extracted from the mud for analysis in the mud logging unit. At present, there is no quantitative correlation between gas levels and pore pressure. However, changes in gas levels can be related to levels of differential pressure in the wellbore indicating over-, near or under-balanced operations. The main sources of gas in the drilling fluid are Gas liberated from the physical action of cutting the rock and circulating the cuttings to surface. Gas flowing into the wellbore due to underbalanced conditions. The gas levels resulting from these sources are dependent on the mud weight, cuttings concentration, differential pressure, ation porosity and permeability and gas saturations. There are three types of gas levels Background Gas, Connection Gas and Trip Gas. Background Gas All drilled wells will have a background gas level which is usually measurable. Background gas originates from the rock being cut by the bit. The background gas level is a function of the porosity of the ation, the hole size column cut, ROP and mud circulation rate. In TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Temperature Data Well Engineering a highly reactive clay mineral. The montmorillonite content is dependent on the level to which montmorillonite conversion to illite has taken place. As discussed in the causes of abnormal pore pressure, with increasing depth and pressure montmorillonite diagenesis to illite occurs, resulting in expulsion of water. In normal compaction trendline, the montmorrillonite content should decrease with depth. Abnormally pressured zones often have a higher montmorrillonite content than normally pressured-shales of the same depth. Hence a plot of the shale factor against depth may show an increase away from the normal compaction trendline on entering the abnormal pore pressure zone. The shale factor would then decrease on leaving the abnormal pressure zone and entering a lower pressure zone. Figure 1.10 Shale Density Trend For Picking Up Overpressured Zones, After Ref 1 Top of overpressures Normal shale trend line Depth Shale Density gm/cc 2.42.62.5 Top of overpressures Normal shale trend line Depth Shale Density gm/cc 2.42.62.5 TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Measurement While Drilling MWD the transit time should decrease with depth due to the decreased porosity and increasing density, Figure 1.11. Abnormally pressured shales tend to have higher porosity and lower density than normally pressured shales at the same depth. Hence the ITT values will be higher. ∆ TOCPreviousNextTOCPreviousNext . . . . . PORE PRESSURE Sonic Logs Well Engineering this is called brittle failure and most rocks fail under this type. Rocks can also de elastically, plastically and then rupture, just like steel. Some rocks exhibit creep effects where the rock des almost indefinitely under the action of a constant stress. The fracture gradient is actually dependent on several factors including ation type, rock strength, mineralogy, permeability and orientation of planes of weakness such as bedding planes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.0FIT PROCEDURAL GUIDELINES The following guidelines outline the typical equipment, preparation and procedures required to per a successful FIT, see Figure 2.4. a Equipment i Mud pressure gauges are not sufficiently accurate for these tests. Large scale gauges of various ratings to cover the expected pressure range of the proposed test should be mounted on a small bore manifold on the cement unit. ii The cement pumps should always be used in preference to the mud pumps when conducting an FIT. b Preparation TOCPreviousNextTOCPreviousNext . . . . . ATION INTEGRITY TESTS FIT Procedural Guidelines Well Engineering the application of a full FIT to leak-off is not recommended. In non-consolidated and loose ations, very low pressures result in leakage of mud to the ation. In this case the final pumping pressure will always be higher than the stabilised pressure. This makes interpretation of fracture gradient in these ation very difficult. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.0PREDICTING FRACTURE GRADIENT When planning exploration or wildcat wells, where there is little or no reliable offset well data, the fracture gradient can be estimated using various predictive techniques. In addition, when drilling a well, fracture gradient should be calculated for each new lithology drilled. It is usually assumed that the fracture gradient at the casing shoe is the lowest within the open hole section. The assumption that the fracture gradient is lowest at the previous casing shoe is not necessarily true. As previously discussed, fracture gradients vary with the in-situ stresses overburden, pore pressure etc., superimposed tectonic stresses and ation type. Hence the lowest fracture gradient in an open hole section is not always found at the previous casing shoe. This can have severe implications for the well control practice and casing setting depths. Fracture gradient estimates made whilst drilling are normally the responsibility of the mud logging contractor. The estimates made should be used to establish a predicted leak-off pressure prior to conducting an FIT. When the predicted fracture gradient is dangerously close to the maximum anticipated mud weight, consideration should be given to pering TOCPreviousNextTOCPreviousNext ATION INTEGRITY TESTS Hubbert and Willis 60 Well Engineering the rock matrix compressibility; the rock bulk compressibility, when and, finally, f PTFBG−− 23 3σσ FBG 3σ3σ2–T 2α 12ν– 1ν– -------------- - – --------------------------------- -Pf– br CC−1α r C b C 15 . 0≤≤α5 . 00≤≤ν 1 1 21 0≤ − − ≤ ν ν α TOCPreviousNextTOCPreviousNext ATION INTEGRITY TESTS Effects of Hole Deviation On ation Breakdown Gradient 62 Well Engineering in this example the highest mud weight is used at TD. 3. Starting at hole TD 11 000 ft, draw a vertical line line 1 through the mud gradient u
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