Detection of Abnormal Formation Pressures Using Drilling Parameters

Transcription

1 Detection of Abnormal Formation Pressures Using Drilling Parameters Ali Ibrahim Mohammed Ameen 1, Prof. Sanjay R. Joshi 2 1ME 2nd year, Department of Petroleum Engineering, Maharashtra Institute of Technology (MIT), Pune, India. 2Professor, Department of Petroleum Engineering, Maharashtra Institute of Technology (MIT), Pune, India *** Abstract - The predication of abnormal formation pressure is a very important factor in designing such wells and it help to avoid many problems through drilling process such as lost circulation which is caused by using excessive mud weight, pipe sticking in hydraulic fracturing operation, blowout and other problems. In wellbore, abnormal Formation pressure could be caused from formation fluid pressure, if it become above or below the hydrostatic pressure. The pressures which be over the hydrostatic pressure referred to an abnormally high pressure or superpressures. While pressures that is below the hydrostatic pressure may be referred to an abnormally low pressures. The current study deals with the estimation of overpressures in selected wells in Southern Iraq. Data used in this study had been obtained from the mud log for West Qurna, well no.15. Predication of overpressures using drilling parameters has been achieved by; Raw penetration rate, d- exponent, dc-exponent and sigmalog. Key Words: Abnormal pressure, Rate of penetration, d- exponent, dc-exponent, Sigmalog, Drilling parameters. as; rate of penetration (ROP), d and dc-exponent, sigmalog,. This calculation depend upon data collected with figures explain the relation of the parameters with depth Pressure Concepts There are a different type of pressure occur during the drilling of any well, as explain below [3] : Hydrostatic pressure: is equal to the vertical height of a column of water extending from the surface to the interesting formation. Abnormal formation pressure: is a variation of interstitial fluid pressure from the hydrostatic pressure of the subsurface fluid. The average total overburden (lithostatic) pressure gradient: resulting from the combined pressure of the rocks (grain-to-grain or rock matrix stress) and their interstitial fluids. Fracture pressure: is the pressure in the wellbore at which a formation will crack. 1. INTRODUCTION In the present-day, drilling and completion operations, cost are the key factors becoming minimum as much as possible in a maximum well control. In order to successfully complete a well, proper well planning and drilling operations are necessary, to minimize the danger of blowouts, stuck pipes, loss circulation, lost hole, and casing setting problems [1]. Many factors can cause abnormal formation pressure, that is, pressure other than hydrostatic. In some area, a collection of these factors are prevails. For example, understanding the importance of petro-physical and geochemical parameters and their relationship to the stratigraphic, structural, and tectonic history of a given area or basin, are very necessary to put the possibility causes of abnormal formation pressures in appropriate perspective. Because conditions can vary widely, distinct attention must be taken not to suppose that the cause of abnormal formation pressure determined through the experience in a well known area is necessarily the cause of a similar condition in a nearby basin, which may not yet have been adequately tested by drilling [2]. 2. Description of the Study The aim of this study is predication abnormal formation pressure through different formation with a different lithology of each formation using drilling parameters such 2.2. Drilling Parameters Raw Penetration Rate The rate of drilling is a function of weight on bit, rotary speed, bit type and size,, hydraulics, drilling fluid, and formation characteristics. Under controlled conditions of constant bit weight, rotary speed, bit type, and hydraulics, the drilling rate in shales decreases uniformly with depth. This is due to compaction increase in shales with depth. However, in pressure transition zones and overpressures, the penetration rate increase. Slower penetration rate is often spotted in the pressure barrier (cap rock) overlying this pressure transition. Also any other main lithological variation in the shales (silty, limey shales, mudstone,..., etc) is reflected in penetration rate variations. Penetration rate should be plotted in (5) to (10) feet increment in slow-drilling formations or (30) to (50) feet increments in fast-drilling intervals. Today, drilling rate recorders are available which automatically plot feet per hour depth. Regardless of how the rate of penetration is recorded, one should establish a normal drilling rate trend while drilling shales in normal-pressure environments for compaction with faster-drilling overpressured shales [2]. Complication can arise due to bit drilling, which may disguise any change in penetration rate due to overpressure [4]. 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2071

2 Normalized Rate of Penetration (d-exponent) The d-exponent was developed to consider changes in the more significant drilling variables to normalize penetration rate. It is derived from the fundamental drilling equation presented by Bingham (1965), which relates penetration rate to weight on bit, rotary speed, bit size and formation drilability: ROP = K ( W / D h ) Nᵉ (1) This equation are simplified by assuming (1) the drilability is relatively constant ( K=1); (2) the rotary speed varies linearly with penetration rate (e=1). Technical accuracy of these assumption is questionable. However, as long as K and N do not vary over too wide range, the results are proportional. Manipulation for field unites conversion, the above equation becomes [4] : ROP = [ ( 12W ) / ( 10 6 D h ) ] 6 N) (2) From thus, the d-exponent can be calculated by: d = [ log ( ) / log ( ) ] (3) Normalized Rate of Penetration (dcexponent) The plot of dc-exponent versus depth is similar to that of d- exponent versus depth. Since the d-exponent is affected by variations of mud weight, an adjustment has been introduced to normalize the d-exponent for the effective mud weight. Mathematically, this cannot be justified, but a plot of dc-exponent versus depth gives a better pictorial presentation than d-exponent. The dc-exponent is calculated from the equation [5] : Sigmalog dc = d. (4) At present, drilling parameters are recorded interpreted, and processed mainly for detection and evaluation of overpressure at the rig site. To obtain this information, AGIP (Azienda Generale Italiana Petroli) applies the sigma log method. The basic equation for sigma log calculation is as follows [6] : = (5) Equation (5) called total raw rock strength equation and is not corrected for depth, it is in fact considered only accurate at 7000 m. If the depth is grater or less than 7000m, a correction must be made, thus: is simply = ( 7 - D/1000 ) (6) with a correction for depth that goes to zero at 7000 m. If all conditions were perfect and the drilling fluid used had exactly the same density as the pore fluid column, then it would be sufficient to plot against depth. This would give a straight line with a slop that reflects increasing compaction and thus rock strength. However, as all bore holes are drilled in over-or-near balanced conditions, depth without taking in to account over balance will result in a curve difficult to interpret. The curve is corrected for overbalance by the following equation, which enables one to plot : Where: = F (7) F = 1 + ( ) (8) The two unknown parameters are thus ΔP and n, where: ΔP = M W - G Pn ) (9) The second unknown is n, which is represent the function of the time required to equalize the differential pressure that exists between the cutting and mud weight. This is dependent on the lithology and the porosity of the cuttings themselves, and is therefore reflected in the value of. - If > 1 (high bit weights and slow drilling): n = ( 4 - ) (10) - If < 1 (low bit weight and fast drilling) [1] : The value of ( n = (11) ), plotted versus depth. As a general trend, this value is always increase with depth through shales, indicating the presence of normally pressured and compacted formation. On the contrary, when the curve of sigma log contains points deviating to the left from the reference trend line as depth increases, it indicates the presence of an over pressured zone [6]. 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2072

3 3. West Qurna Oil Field (WQ - 15) It is existed about 70 km Northwest of Basra city in southern Iraq. West Qurna is one of the biggest oil fields in Iraq. This deep well was the fifteenth wells drilled by the Iraqi National Oil Company in West Qurna field of Southern Iraq. West Qurna 15 was the first well that has been drilled near the crest of West Qurna structure. It is extend from Upper Fars formation at surface to final depth at Najmah formation at 4400 m. This field contains a certain reserves predestined at 18 billion barrels and reserves potential is estimated at 40 billion barrels. Now, the production of the field is about 120,000 bbl/day, but it can reach to 1 million bbl /day. It is one of the oils light desired globally, the bottom hole pressure is around 7200 psi and the number of oil wells is 247 wells, while the number of water injection wells is 64 wells [3] Lithology of (WQ - 15) The formation of this field and its composition which are covered in this study according to availability of data are explain through the table below [7] : Table-1: Formation, depth range and lithology of (WQ- 15). Formation name Depth rang m Lithology Tanuma 2170 Black fissile shale, macro crystalline, argillaceous, detritus limestone Khasib 2218 Chalky, oligsteginal limestones Mishrif 2270 Organic detritus limestones, beds of algal, coral-reef limestone and limonitic fresh water limestone Rumaila 2432 Oligsteginal limestones, beds of dolomite, dolomitic limestones Ahmadi 2513 Gray shale, Limestone Mauddud 2648 Dolomitize organic, detritus limestone Nahr Umr 2803 Black shales, grained sandstones, amber, pyrite Shuaiba 2991 Shaly limestone Zubair 3081 Sandstone, siltstone, alternating shale Ratawi 3406 Slightly pyritic, shales, beds of buff, pseudo oolitic, detritus limestones Yamama 3529 Argillaceous limestones, oolitic shoal limestones Sulaiy 2857 Marly limestone, oolitic limestone Gotnia 4120 Calcareous shales and salt Najmah 4400 Shale with streaks of limestone 3.2. Pressures Distribution in (WQ-15) Formation The formation with an abnormal pressure in (WQ-15), are explain below [7] : - Shuaiba Formation (Subnormal Pressure): Consider as one of a critical formations with big problems like of the loss of drilling fluid circulation (total loss), as well as stuck pipe and hole failed problems. - Yamama Formation (Abnormal Pressure): contain oil and gas in two carbonate units each one with basal reservoir oolitic limestones overlain by lime mudstone seals. - Sulaiy Formation (Abnormal Pressure): The problem of this formation is the appearance of gas with high pressure that causes the abnormal high pressure with flow of gas inside the well. - Gotnia Formation (Abnormal Pressure): Numerous states of flowing saltwater and gas kicks have been occurred. - Najmah Formation (Abnormal Pressure): the flow of gas or salt bed (Abnormality high pressure) problems. 4. Calculation and Results with Discussion 4.1. Rate of Penetration According to availability of data, raw penetration had been drawn versus depth to give allusion of the occurrence of overpressure zone as shown in figure (1) and table (2), which show that the rate of penetration increased slightly in Yamama and Sulaiy formation. While at nearly (4200 m) depth, we note a high rate of penetration, due to the effect of Gotnia salt formation which it is denser than compared with other rocks. Penetration rate in other formation is lower due to two reasons: 1. The bit show dulling tendencies with depth which naturally decreases penetration rate throughout the bit life. 2. With an increase in depth, a constant mud density will result an increase differential pressure across the borehole, if the pore pressure remains normal. 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2073

4 Table-2: Calculated values of d and dc-exponent at selected depth. Fig- 1: Rate of penetration versus depth relationship Normalized Penetration Rate (d-exponent): Because of the fact that many variables effect upon penetration rate, so d-exponent was used to normalize these variables. Equation (3) was used to calculate d-exponent values from drilling data which includes (penetration rate, rotary speed, bit diameter, weight on bit, and mud weight). Example of this calculation shown as below: - At depth 4025 m ROP (ft/hr) = 0.677, WOB (1000 Ib) = 18, N (RPM) = 70, Dh (in) = d = [ log ( ) / log ( ) ] - d = Table (2), shows these calculation at selected depths. D- exponent had been plotted versus depth in figure (2). This plot shows an increase in d-exponent values in normal pressure zones, while at high pressure zones (Yamama, Sulaiy, and Gotnia formations), d-exponent values will decrease. This decreasing in d-exponent may be due to the effeteness of the lithology of this formations that make a change in the shale section, such as ( silty shales, mudstones, limey, marls, etc.) Normalized Penetration Rate (dc-exponent): Since d-exponent method is influenced by mud weight variations, dc-exponent method will normalize d-exponent values for the variations of mud weight by using equation (4). - For example, at depth, D = 4025 (m), d-exponent = , Mw n = 1.13 (gm/cc), Mw = 2.09 (gm/cc) - dc = * - dc = The resulted values are tabulated in table (2), and plotted versus depth in figure (3). This plot shows an increase in dcexponent values in normal pressure zones, while at overpressure zones, dc-exponent values will decrease due to the increase of the rate of penetration. 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2074

5 4.4. Sigmalog Method: Interpretation of sigmalog method is based on computing rock strength values ( o) from drilling data which includes (penetration rate, rotary speed, bit diameter, weight on bit, and mud weight). We started with equation (5) to find the total rock strength (raw sigmalog) ( t). Which is not corrected for depth, therefore equation (6) used to corrected this values in a selected depth. Equation (7) used to find the value of Sigma log ( o). But in this equation there is a one parameter called the overbalance correction (F) which it found from equation (8). The value of Δp) differential pressure will be found from equation (9). And the value of (n) which represent the time required to equalize the differential pressure by using equation (10). Example of this calculation shown as below: - At depth, D = 4050 (m) = ( ) / ( ) = psi - Corrected rock strength = ( / 1000 ) Fig-2: d-exponent versus depth relationship. = psi - From Eq (10), find the value of n-function: n = ( 1 / 640 ) [ 4 - ( 0.75 / ) ] n = Eq (9), used to find the value of differential pressure Δp): Δp = [ ) - ( )] ( 4050 / 10) Δp = psi - To find the values of overbalance correction (F), used Eq (8) F= 1 + [ ] F = Finally, the values of Sigma log found from Eq (7) = = psi Fig-3: dc-exponent versus depth relationship. The value of ( o) and it calculation are tabulated in table (3). The plot of ( o) versus depth had been created in figure (4). Sigmalog plot is similar to d-exponent and dcexponent plots, where ( o) values increase with depth, showing normally pressured zones, while in abnormally 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2075

6 pressured zones ( o) values decrease from the reference normal trend line. For the raise of sigmalog plot, shifts of the normal compaction trends will be created by drawing the best line through data points for each segment, this is shown in figure (4). The shifts of sigmalog plot may be due to many causes: 1. Geological cause, such as faults,...etc. 2. Drilling causes, such as improper bit selection,... etc. Table-3: Calculated values of sigmalog at selected depth. Fig-4: Sigmalog versus depth relationship. 5. Conclusions From figure (1),rate of penetration (ROP) increased slightly in Shuaiba formation because, this formation is consider as an subnormal pressure which is represent a one of critical formation with different problems such as loss circulation, or stuck pipe. Again a slights increase appear in Yamama and Sulaiy formation, due to presence of gas with high pressure. But a high rate occur on Gotnia formation because the affect of salt which it is denser than the other rocks. In Najmah formation, ROP will started to decrease but still there is a sign of abnormal pressure because the effeteness of gas and salt formation. In a normalized penetration rate (d-exponent and dcexponent) and sigmalog method which shown in figures (2,3 and 4). The values of this techniques decreases in a high pressure zone (Yamama, Sulaiy, Gotnia and Najmah formation) due to increase (ROP). And due to the effeteness of the lithology of this formation that make a change in the shale section. 2017, IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2076

7 NOMENCLATURE BIOGRAPHIES ROP - Rate of penetration (ft/hr) K - Formation drilability D h - Bit diameter (in) d - Exponent for normalizing penetration rate N - Rotary speed (RPM) W - Weight on bit (Ib) dc - Corrected d-exponent MW N - Normal mud weight gradient (gm/cc) MW A (M W) - Mud weight (gm/cc) - Total rock strength or raw sigmalog (psi) - Corrected total rock strength (psi) D - Depth (m) - Rock strength (psi) F - Overbalance correction ΔP - Differential pressure (psi) G Pn - Normal formation pressure gradient for the area (psi) Ali Ibrahim Mohammed Ameen is a second year master of engineering (ME), student in Department of Petroleum Engineering at Maharashtra Institute of Technology (MIT), Pune, Maharashtra, India. Prof. Sanjay R. Joshi, professor in Department of Petroleum Engineering at Maharashtra Institute of Technology (MIT), Pune, Maharashtra, India. ACKNOWLEDGEMENT A special thanks to may department (Department of Petroleum Engineering) and professor (Dr. Pradeep B. Jadhav) Head of the Department to his support. Thanks and appreciation to my guidance professor (Sanjay R. Joshi) who has given me his whole potential in giving me a way to obtain the goal as well as his encouragement to maintain progress in course. REFERENCES [1]. Bhagwan Sahay, Walter H. Fertl, "Origin and Evaluation of Formation Pressures", Allied Publishers Private Limited, New Delhi [2]. Walter H. Fertl, George V. Chilingarian, and Herman H. Rieke, "Abnormal Formation Pressures: Implications to Exploration, Drilling, and Production of Oil and Gas Resources", Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York [3]. Ahmed Kareem Hassan, " A Study of Abnormal Formation Pressures Distribution and Their Effect on Drilling Operation in Middle & South Iraqi Oil Fields", Petroleum Engg. Dept. Baghdad University, [4]. J. R. Jorden and O. J. Shirley, "Application of Drilling Performance Data to Over-Pressure Detection", JPT, Nov. 1966, pp. ( ). [5]. Bill Rehm and Ray McClendon, "Measurement of Formation Pressure from Drilling Data", SPE No. 3601, [6]. P. Belloti and D. Giacca, "AGIP Technology; in Deep Drilling, Pressure Evaluation and Drilling Performance", AGIP, Milan, Italy, [7]. Saad Z. Jassim and Jeremy C. Goff, "Geology of Iraq", First Edition, , IRJET Impact Factor value: ISO 9001:2008 Certified Journal Page 2077