Energy and Sustainability V579Pore, abnormal formation andfracture pressure predictionF. Khoshnaw, P. Jaf & S. FarkhaPetroleum Engineering Department, Koya University, IraqAbstractOil and gas well drilling planning is the main task of any drilling engineer. In orderto overcome this task, it is required to maintain the wellbore pressure between themaximum value that does not fracture the formation and the pressure of the fluidswithin the formations, because, as the wellbore pressure exceeds the fractures’pressure, formation damage occurs, which will consequently result in lostcirculation problems. Meanwhile, if the well pressure is less than the formationfluids pressure it leads to other drilling problems such as the kick problem andpossibly blowout. The maximum well pressure that does not fracture theformations is called the fracture pressure and the formation fluids pressure is calledthe pore pressure. Therefore, pore pressure and fracture pressure are considered asthe most crucial parameters for drilling engineering planning and for launchingnew wells. Another significant parameter for well planning is the detection andestimation of an abnormal pressure depth. This is when the formation pressure ishigher than the normal pressure at a specific depth.There are many methods for determining pore pressure, fracture pressure andabnormal pressure depth and their values. These methods can be divided into twocategories: predictive methods and verification methods.In this paper, the predictive method involves determining or estimating theabove-mentioned parameters prior to drilling operation using seismic data,especially equivalent to matrix stress. This is applied to two case studies: anAfrican sandstone reservoir in Libya and the South Texas Frio Trend, based onseismic data recorded for interval transit times vs. depth. The results showed that;first, for the African sandstone reservoir, Libya, the oil filed the abnormal porepressure located deeper than 6000ft, which is the main indicator for specifying thatthe maximum depth has an abnormal pressure. Second, for the South Texas FrioWIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)doi:10.2495/ESUS140511

580 Energy and Sustainability VTrend, the abnormal pore pressure was located at deeper than 8000ft. In the lastsection, based on the pore and fractured gradient, the casing sets are selected.Keywords: pore pressure, abnormal pressure, fracture pressure, pore andfracture pressure gradient, prediction.1 Introduction1.1 Concept of pressurePressure is the ratio of force to the specific surface area over which a load ispressed. For instance, solids exert pressure; the most interesting examples ofpressure in this paper involve fluid pressure, which may be gases and/or liquids,and/or water and air in particular cases within the subsurface of the earth. Pressureplays significant roles within the petroleum industry, among these is its functionin the operation of lifting fluids from the subsurface while in upstream operations.The maintenance of ordinary formation pressure is essential to oil and gasproduction; the oil fields are perfectly suited to the ordinary pressure of thereservoir pressure, and that pressure is altered significantly and periodicallythroughout the time of production [1].Within the oil and gas reservoirs there are some explicit pressure terms whichshould be analysed and then applied to a real case study from Sirt, Libya. Thoseterms are: pore pressure or formation pressure. They are defined as the pressurepressing on the fluids inside the pore space of a formation. For more clarification,overpressure or geopressures or abnormal pressures are defined as any pressuresencountered which are greater than the normal fluid pressure. In addition to theirdefinition, mathematical correlations are derived to encounter their value, forexample, hydrostatic pressure, Phyd, is the pressure caused by the weight of acolumn of fluid:where , ρf and g are the height of the column, the fluid density, and accelerationdue to gravity, respectively [2].1.2 Borehole environmentThe situation of boreholes and the down status of oil and gas wells should beconsidered before launching any drilling operation. Thus, while planning anddesigning a well to be drilled it is crucial to consider the hydrostatic pressure ofthe drilling fluid, because if the pressure of the mud column above the pore zonesis less than the pore pressure, the well is considered underbalanced. Eventually,sections of this well can collapse inside the wellbore, formation fluid can flowinto the well from the surrounding rock and an uncontrolled flow of oil and gasinto well is called a kick. In the extreme case, if the kick is not considered or dealtwith, then that kick can lead to a blowout. As a result of the kick, the rock particleswill fracture and break inside the well.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

Energy and Sustainability V581On the other hand, if the hydrostatic pressure of the mud column is higher thanthe formation pressure, the well is considered as overbalanced. In unconsolidatedformations, i.e. when the formation layers are carbonate rocks such as limestoneand sandstone as shown in figure 1, this will force drilling mud to penetrate intothe formation, thus causing mud filtrate within the zone of interest. Results fromthe overbalanced method are shown in figure 1, through which a damaged zonewill be created and the degree of severity of the mud filtrate is illustrated in thesame figure as follows [3]:A. Invaded zone, the formation fluid is completely displaced by the mud filtrate;B. Transition zone, the formation fluid is partially displaced by the mud filtrate;C. Uninvaded zone is the zone which is not affected by the drilling fluid andalso is called virgin zone.Figure 1:Borehole environment during drilling [3].2 Literature review2.1 Fracture pressure conceptFracture pressure is the pressure in the wellbore at which a formation will crack.Fracture pressure may decrease with decreasing reservoir pressure. Hereby, it ishabitually necessary to carry out “breakdown” tests to conclude the fracturepressure of a specific zone of an oil or gas reservoir. Figure 2 illustrates pressurebehavior during a test to determine fracture pressure.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

582 Energy and Sustainability VFigure 2:Test to determine fracture pressure [1].The test procedure is to start pumping water or clean oil into the formation at avery slow rate, perhaps ¼ to ½ bbl per minute for short zones, and measure thepump pressure. Then, increase the pump rate by steps and record the injectionpressure until the injection rate–pressure curve breaks, as indicated in point B offigure 2. The stress within a rock can be resolved into three principal stresses. Aformation will fracture when the pressure in the borehole exceeds the least of thestresses within the rock structure. Normally, these fractures will propagate in adirection perpendicular to the least principal stress [4].Figure 3:Pressure versus depth correlation [4].2.1 Matthew and Kelley correlation for fracture pressure predictionMatthews and Kelley replaced the assumption that the minimum stress was onethird the matrix stress by: min F mawhere the stress coefficient was determined empirically from field data that istaken from normally pressured formations.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

Energy and Sustainability V583The vertical matrix stress at normal pressure is calculated (subscript “n” is fornormal pressure):(sma)n sobn – PfnFor simplicity, Matthews and Kelley assumed that the average overburdenstress is 1 psi/ft and an average normal pressure gradient is 0.465 psi/ft. Tocalculate abnormal fracture pressure, they introduced the depth Di. Di is theequivalent normal pressure depth, which represents the abnormally pressuredformation of interest depth.( ma ) n 1 Di 0.465Di 0.535DiAt the depth at which the abnormal pressure presents:Di ( ma ) n ob Pf D Pf 0.5350.5350.5352.2 Abnormal pressureAbnormal pressure is diagnosed for a subsurface condition in which the porepressure of a geologic formation exceeds or is less than the expected, or normal,formation pressure. When an impermeable layer of rocks such as shales arecompacted quickly, fluids within their pores cannot continuously escape and thuswill be added to the total overlying rock column, resulting in abnormally highformation pressures. Eventually, excess pressure, called overpressure, can cause awell to blowout or become uncontrollable during drilling. Severe under-pressurecan cause the drilling strings to stick to the under-pressured formation.2.3 Pore pressurePore pressure, or the pressure of fluids within the pores of a formation, is animportant assessment that must be carefully made when planning a drilling project.Formations with pressures higher than hydrostatic pressures can be encounteredin varying areas and depths. Being unaware of areas with overpressures can createmany potentially catastrophic events such as blown reservoir seals, drilling fluidlosses, or formation fluid influxes. There are many causes of overpressure; thus, itis vital to take the time to strategically plan, analyse, and model pore pressures andfracture pressures as accurately as possible.2.4 Importance of pore pressure calculationsTwo of the most important parameters in designing and drilling a well for oil andgas are the wells’ pore pressure gradients and fracture gradients. This paper willshow how pore pressure and fracture gradients change as a well is drilled deeperinto the earth and how drilling engineers use drilling mud to manage the subsurfacepressure. Pore pressure is the pressure which is exerted by fluids within the porespaces of rocks. That pressure is generated by the rock in the fluids above the poreWIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

584 Energy and Sustainability Vzones. When drillers encounter the zone of pores, which is hydrocarbon bearingrock, the pore pressure will force oil and gas out of the rock into the well, unlessthe drillers do something to counteract that pressure.Drillers use drilling fluids, including drilling mud to counterbalance the porepressure, through adjusting the column of the mud weight inside the well so that apressure will be generated to the bottom of the well that equals to the pore pressurewithin the rocks. Consequently, the well will be kept in balance. Drillers measurethe pore pressure in terms of the density of the mud column that would be requiredto balance that pressure. A plot of that pressure as shown in figure 4 can beexpressed in terms of pressure units as a function of depth is called the porepressure gradient [2].Figure 4: Pore pressure gradient [2].2.5 Estimation of formation pressuresHottman and Johnson developed a technique based on empirical relationshipswhereby an estimate of formation pressures could be made by noting the ratiobetween the observed and ordinary rock resistivities. As it is explained in theirstudy, the following steps are required to evaluate the formation pressure:1. The normal trend is established by plotting the logarithm of shale resistivityvs. depth.2. The top of the pressured interval is found by noting the depth at which theplotted points diverge from the trend.3. The pressure gradient at any depth is found as follows:a) The ratio of the extrapolated normal shale resistivity to theobserved shale resistivity is determined.b) The formation pressure corresponding to the calculated ratio isdetermined using figure 5.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

Energy and Sustainability V585Figure 5: Shale resistivity from the log [4].2.2 Pore pressure prediction from seismic dataAnother method of estimating pore pressure is using seismic data collection.Through this method the advancement of seismic processing allows anincreasingly accurate estimation of the velocities. This method allows theengineers and geoscientists to combine their knowledge and information toproduce a valuable method for pore pressure prediction.In short, the process of deriving accurate possible velocities from seismicprocessing is one of the key factors in predicting reliable pressure. As a result, thisis where the role of the geophysicist comes into play in the process of pressureprediction. On the other hand, the geophysicist’s information needs to becombined with an in-depth analysis of the well data, pressure data, and drillingdata by an experienced person such as petrophysicist and drilling engineer. Inaddition to the engineering data and geophysicist’s information, another importantsource of data to be gathered is from the role of the reservoir geophysicist for theintegration of subsurface information with the specially conditioned seismic datato obtain reliable results [4].3 MethodologyIn the previous sections several methods, which are used to estimate and predictthe pore pressure, have been described. In this section the equivalent matrix stressmethod will be used to predict the pore pressure and to find out both the porepressure and the fracture pressure gradient.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

586 Energy and Sustainability V3.1 Equivalent matrix stressThis method can be applied by using seismic record data for Well Cat exploratorywells also for the sonic log data of the drilled section of the well, when the openhole logging tools are available. The procedure to estimate the pressure from thelog-derived interval transit time vs. depth is the same and uses data either from aseismic record or a sonic log. The only difference is when shale formation isincluded in the analysis. The main difficulty in using seismic record data islithology because it cannot be determined accurately; therefore, the averageinterval transit time for all formations must be present. However, it is oftendifficult to detect a sufficient number of shale points in the shallow pressuredformation to establish a normal trend line with the data for a single well, as shownin figure 6.Based on figure 6, the observed normal pressure trend line is drawn throughthe normal pressured zones, but at some point, the observed pressure is the samevalue as the above pressure point, which indicates an abnormal pressure formation;this is under the same matrix stress (grain-to-grain stress) [5]. For example, thematrix stress σg of an abnormally pressured formation at depth D, which is thesame matrix stress σ gn of a more shallow normally pressured formation at depthDn, indicates the same porosity dependent parameter value t.Figure 6: Porosity dependent parameter vs. depth [8].At every depth point within the earth’s subsurface, rocks are simulated bya piston being forced in equilibrium because of the balance of the forcesacting upward and downward on these rocks. Based on the above expression;the overburden pressure is the relation between the matrix pressure and theformation pore pressure, as follows [6]: ob g PfWIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

Energy and Sustainability V587where: ob total overburden pressure resulting from the geostatic load above, psi; g grain-to-grain pressure, psi;Pf fluid pressure inside the pores, psi.Therefore, for the matrix stress of abnormal and shallow pressured formations,the above equation can be written as the equivalent depth: ob (ab) g (ab) Pf (ab) ob (n) g (n) Pf (n)As previously mentioned, at both equivalent depths the matrix stresses arethe same; therefore, the above equation can be written as: g (ab) g ( n )3.1.1 Pore pressure gradient determinationA relation between pore pressure and related depth can be expressed as:pore pressure gradient pore pressuredepthThus;Gf pfDThe pore pressure gradient can be solved simultaneously by the above twoequations to give the following equation. The below equation is used to determinethe formation pore pressure gradient at the selected depth D:Gf Gob ab D G f n Gob n DeDwhere: ob (n) The total overburden pressure, psi; ob (ab) The total overburden pressure, psi; g (n) The normal grain to grain pressure, psi; g (ab) The abnormal grain to grain pressure, psi;Pf (n) The normal fluid pressure, psi;Pf (ab) The abnormal fluid pressure, psi;Gob (n) The total overburden pressure gradient, psi;Gob (ab) The total overburden pressure gradient resulting from the abnormalgeostatic load, psi;Gf (n) The normal fluid pressure gradient, psi;Gf The actual fluid pressure gradient, psi;De Equivalent depth at the shallow formation, ft;D Actual depth of interest, ft.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

588 Energy and Sustainability VThe overburden pressure gradient Gob can be obtained from the followingequation:Gob 0.433 bwhere: b Rock bulk density read from density log, gm/cc;0.433 Conversion factor from gm/cc to psi /ft.3.1.2 Fracture pressure gradient determinationTheoretically the fracture gradients can be determined if the geologicalconditions of the formation are known, including: Depth of the zone of interest; Formation fluid pressure within that zone; The rock type, i.e. sand, shale, etc.There are some theoretical methods of determining the formation fluidpressure gradients and the formation fracture gradients such as Hubbert, Eaton,Mathews and Kelly.As it has been described, the fracture pressure gradient should be knownbefore drilling the wells in order to avoid fracturing of encountered formationsand hence prevent lost circulation due to the wrong selection of mud weights.Consequently, the relation between the bottom hole pressure and the formationfracture gradient should be found in order to determine when and where a casingstring set might be needed.In this section, two correlations will be described to determine the fracturepressure gradient:1. Hubbert and Willis (1957) suggested a correlation for the determination of thefracture gradient based on overburden pressure and formation pore pressure.2P 1 FG ob f 3 DD 2. Eaton (1969) also proposed a correlation to give accurate results compared tothe other methods and it is presented as follows: ob Pf Pf FG D D 1 Dwhere:FG Fracture pressure gradient, psi/ft; Poisson’s ratio;Pf Formation pore pressure, psi; ob Overburden pressure, psi.The overburden pressure gradient is calculated using the density log readingsas follows: ob 0.433 bDwhere b Average rock bulk density read from density log.WIT Transactions on Ecology and The Environment, Vol 186, 2014 WIT, ISSN 1743-3541 (on-line)

Energy and Sustainability V5894 Case studies4.1 Case study ASeismic data was recorded from the African sandstone reservoir, Libya, as givenin table 1. The normal hydrostatic formation pore pressure obtained from thesalinity data for the area is 0.449 psi/ft. The average overburden pressure obtainedfrom a density log in a nearby similar area is estimated to be 1.0 psi/ft.Table 1: Seismic data of African sandstone reservoir, Libya.Averagedepth ft200030004000500060007000Average intervaltransit time (10-6 1,00012,000Average intervaltransit time (10-6 s/ft)1201261311301304.2 Case study BSeismic data are recorded from the South Texas Frio Trend, as shown in table 2