Calculation analysis of sustained casing pressure in gas wells

Petroleum Science, Jan 2012

Sustained casing pressure (SCP) in gas wells brings a serious threat to worker safety and environmental protection. According to geological conditions, wellbore structure and cement data of gas wells in the Sichuan-Chongqing region, China, the position, time, environmental condition and the value of SCP have been analyzed. On this basis, the shape of the pressure bleed-down plot and pressure buildup plot were diagnosed and the mechanism of SCP has been clarified. Based on generalized annular Darcy percolation theory and gas-liquid two-phase fluid dynamics theory, a coupled mathematical model of gas migration in a cemented annulus with a mud column above the cement has been developed. The volume of gas migrated in the annulus and the value of SCP changing with time in a gas well in Sichuan have been calculated by this model. Calculation results coincided well with the actual field data, which provide some reference for the following security evaluation and solution measures of SCP.

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Calculation analysis of sustained casing pressure in gas wells

Pet.Sci. Calculation analysis of sustained casing pressure in gas wells Zhu Hongjun 1 Lin Yuanhua 1 Zeng Dezhi 0 Zhang Deping 0 Wang Feng 0 0 Jilin Field Company , PetroChina, Songyuan, Jilin 138000 , China 1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University , Chengdu, Sichuan 610500 , China Sustained casing pressure (SCP) in gas wells brings a serious threat to worker safety and environmental protection. According to geological conditions, wellbore structure and cement data of gas wells in the Sichuan-Chongqing region, China, the position, time, environmental condition and the value of SCP have been analyzed. On this basis, the shape of the pressure bleed-down plot and pressure buildup plot were diagnosed and the mechanism of SCP has been claried. Based on generalized annular Darcy percolation theory and gas-liquid two-phase uid dynamics theory, a coupled mathematical model of gas migration in a cemented annulus with a mud column above the cement has been developed. The volume of gas migrated in the annulus and the value of SCP changing with time in a gas well in Sichuan have been calculated by this model. Calculation results coincided well with the actual eld data, which provide some reference for the following security evaluation and solution measures of SCP. Sustained casing pressure; gas migration; coupled mathematic model; gas well 1 Introduction After well completion, pressure in all of the casing strings should be zero if the well is owing at steady state conditions, but due to initial thermal expansion effects a small volume of uids has to be bled through a needle valve in order for the casing pressure to fall to atmospheric pressure (Bourgoyne et al, 1999) . If the casing pressure builds up when the needle valve is closed, the casing is said to exhibit sustained casing pressure (SCP). Well cement problems such as small cracks or channels can result in gas migration and lead to SCP at casing heads (Dusseault et al, 2000; Kinik and Wojtanowicz, 2011) . In some cases, the casing pressure can reach dangerously high values. SCP in acid gas wells brings a serious threat to worker safety and environmental protection. Therefore, we need to better understand the reason for gas migration and the mechanism of SCP. 11,498 casing strings in 8,122 wells in the Gulf of Mexico (GOM) have been reported with SCP (Bourgoyne et al, 1998) . Studies funded by the MMS (US Minerals Management Service) have been done by some investigators (Wojtanowicz et al, 2001; Xu, 2002; Xu and Wojtanowicz, 2001; 2003) . Based on previous research, MMS regulations (30 CFR 250.517) require elimination of SCP and grant departures permitting operation of wells with small SCP problems. However, wells with approved departure must be frequently tested in order to monitor and control the severity of SCP. These tests include pressure bleed-down and pressure buildup. The diagnostic test calls for bleeding the pressure to zero through a 0.5-in needle valve and recording the casing pressure. Then the value of initial pressure bled down during the test can be obtained from the recorded data. Recorded pressures from other annuli would indicate whether there is communication between different casings in the well. However, no analytical method has been developed for quantitatively analyzing these tests. The needle valve is closed to initiate pressure buildup and the pressure recorded for 24 hours. The rate of pressure buildup could provide additional information about the size and possibly the location of the leak. But there is still no method for interpreting the test. Furthermore, testing of SCP is mostly qualitative and limited to arbitrary criteria. Such information is insuf cient for operators to quantitatively analyze SCP problems. Thus there is a need for improved analysis that could provide more information. The work presented here focuses on the mechanism of SCP and a coupled mathematical model of gas migration in a cemented annulus with a mud column in a gas well, which provide some reference for the following security evaluation and solution measures of SCP. 2 Field data analysis SCP is a universal problem occurred in gas wells in China. The field data are casing pressure records provided by various operators from 13 gas wells, which mainly come from the Longgang, Luojiazhai, Puguang, Zhongba and Moxi gas elds in Sichuan-Chongqing region in China. 2.1 Statistical analysis of SCP data Of the 13 gas wells, 12 show SCP problems (shown in Table 1) and all the production casing strings exhibit SCP. The percentage of SCP presenting in intermediate casing strings, surface casing strings and conductor casing strings are 90%, 83.3% and 7.7%, respectively. Fig. 1 shows the frequency of SCP for different casings. Production casings and intermediate casings present more serious SCP. Fifty percent of the production casings and 56 percent of the intermediate casings have SCP of less than 10 MPa. And 80 percent of the surface casings have SCP of less than 5 MPa. The SCP magnitude in conductor strings is the lowest, all have SCP of less than 5 MPa. For instance, the productive reservoir of A gas field is in the Feixianguan Formation. After well completion, the gas production rate of the A1 well reached 114×104 m3/d. However, the casing pressure in the production casing string is 43.5 MPa, which indicates serious gas migration in the annulus. The location of the mud surface in the annulus is at a depth about 200 m. The possible reasons leading to SCP are a poor cementing job, pressure fluctuation, and tubing leak. In some cases, the tubing leak even presents above the mud surface. The production casing pressure in the A2 well is 23 MPa, and the intermediate casing pressure is 8.4 MPa. Intermediate casing pressure in the A3 well is 12 MPa. A major cause of the SCP problem in the two wells, besides poor quality of cement, is casing leakage. Notes: Y-SCP problem; N-no SCP problem; NA-data not available. <10 MPa <10 MPa Production casing Intermediate casing Surface casing Conductor 2.2 SCP typical patterns Five typical response patterns (Xu, 2002; Milanovic and Smith, 2005) could be concluded from the eld data of 13 gas wells, which include two SCP bleed-down patterns and three SCP buildup patterns: Instant bleed-down pattern Long bleed-down pattern Normal buildup pattern S-shape buildup pattern Incomplete buildup pattern The bleed-down pattern depends on the opening of the needle valve controlled by operators and the amount of uids bled from the casing annulus. If the needle valve is opened wide to bleed a small amount of gas and liquid from the casing annulus, the casing pressure would drop to 0 in a very short time, named as instant bleed-down pattern. On the other hand, if operators manipulate the needle valve to minimize the removal of fluids, the duration of bleed-down would be prolonged by the operation, so the casing pressure may not decrease to 0 over the duration of the bleed-down test. It is the other pattern, the long bleed-down pattern. The two bleeddown patterns are shown in Figs. 2 and 3. Removed 0.15 m3 1.21 g/cm3 fluid 12 15 Time, sec 3 6 9 18 21 24 27 Non-linear 0 3 6 12 15 18 9 Time, min As shown in Figs. 4, 5 and 6, we can see three buildup patterns, which depend on operating conditions such as the magnitude of gas migration, the duration of bleed-down, and the mud weight. After the bleed-down, a normal buildup pattern is observed when a quick initial pressure increase is followed with a transition period of gradual increase until it comes to pressure stabilization. The stabilized casing pressure depends on the mud weight and gas-source formation pressure. The transition period is determined by the magnitude of gas migration in the cement and mud column. If there is almost no gas left in the mud column after bleeddown, there would be no obvious increase in the casing pressure until the rst pulse of gas reaches the casing head. Then the casing pressure will increase gradually and nally stabilize. This is the S-shape buildup pattern. An incomplete buildup pattern is noted where no late-time stabilization is apparent in the testing interval (usually 24 hours) and the initial casing pressure increase in the early time is relatively low. Transition time 5 10 15 Time, h Fig. 4 Normal buildup pattern e m it e t a L 20 25 10 8 a P M ,re 6 u s s e r gp 4 n i s a C 2 0 0 9 a 8 P M , e r u sse 7 r p g n i s aC 6 e m it y l r a E e m it y l r a E Transition time e m it e t a L 5 10 15 20 25 Time, h Gas wells are composed of many layers of casing strings, which constitute several annuli. According to the location, the annulus from inside to outside can be named “A” annulus, “B” annulus, “C” annulus, and so forth (Anders et al, 2006) . As shown in Fig. 7, “A” annulus represents the annulus between the tube and production casing. “B” annulus represents the annulus between production casing and adjacent intermediate casing. The rest can be obtained by analogy. As shown in Fig. 8, two possible configurations of the cement column in the annulus are common: cemented to the surface or a mud column above the cement. A gas cap may be present above the mud column. In wells cemented to the surface, gas migration can be considered as a one-dimensional flow through a medium of some conductivity (Nishikawa, 1999) . After bleed-down, the buildup behavior is controlled by cement properties, such as permeability and porosity, and by the gas formation pressure. While in wells with a mud Fig. 11 Possible faults leading to SCP in the B or C annulus Hanger failure Packer failure Valve failure Cement leak Corrosion leak Connector leak where p is the pressure in the cement, MPa; z is the distance from the gas-source formation, m; t is time, s; k is the cement permeability, m2; Lc is the length of the cement column, m; g is the gas viscosity, Pa·s; is the cement porosity, dimensionless; and cg is the gas compressibility; Pa-1. The boundary and initial conditions may be stated as follows: a) p=pc at t=0 for z (0<z<Lc) b) p=pf at z=0 for all t p q c) g g for t>0 z z Lc kA where pc is the pressure on the top of the cement, MPa; pf is the reservoir pressure, MPa; qg is the gas ow rate, m 3/s; and 2 A is the wellbore area, m . 4.2 Gas migration in the stagnant mud Gas migration in the stagnant mud can be modeled as dispersed two-phase flow that can be described by a driftux model (Xu and Wojtanowicz, 2003) . Basic assumptions are a concentric annulus, equal phase pressure, uniform phase Gas cap Gas bubble Mud Cement Gas migration channel Formation pressure in the cement can be obtained as: z=0 pf=const 2 p z2 2 p z2 with p 2 z 1 p t 1 p 1 p t k g cg if p 15MPa and z Lc if p 15MPa and z Lc (1) (2) densities normal to the ow direction, constant temperature pro le, and thermodynamic equilibrium. Due to slow phase segregation after the bleed-down, it is assumed that the relative velocity term is negligible. Under the assumptions, the one-dimensional two-equation drift-flux model is summarized in the following (Santos and Azar, 1997; Xu, 2002) . Continuity equations for the continuous (mud) and the dispersed phase (gas) are: ) L (1 ) LvL 0 ( gvg ) z z 0 where is the void fraction; g and L are the gas and liquid density, kg/m3; and vg and vL are the gas and liquid velocity, m/s. And mixture momentum equation is: v 2 m m z p z m g f 2dh mvm2 0 (5) ( (1 t t g ) mvm t vs (qg do g with vg vm dh m C v 0 m di qL ) / A vsg vsL L (1 ) where m and vm are the mixture density (kg/m3) and velocity (m/s), respectively; f is the friction factor; C0 is the distribution factor; dh is the hydraulic diameter of the annulus, m; di and do are the diameter of the inner string and the outer string, respectively, m; vs is the gas slip velocity, m/s; vsg and vsL are the supercial velocity of gas and liquid, respectively, m/s; and qL is the liquid ow rate, m 3/s. In order to complete the model, the slip velocity (vs) and the fluid fraction factor (f) must be algebraically specified. Based on analysis of the SCP field data, it can be safely assumed that the ow pattern in annulus is either bubble or slug ow. According to the Hasan and Kabir method (Hasan and Kabir, 1993) , the value of the distribution factor C0 for bubble ow can be described as: where m is the mixture viscosity, m/s; and Rem is the Reynolds number of two-phase ow. The mixture viscosity for two-phase ow is: L L g (1 L ) Re v d AL Ag AL Re (11) (13) (14) (15) (16) (17) (18) (19) (3) (4) (6) (7) (8) (9) with with f Re m L f (1 (1 ( ( where L is the mud viscosity, Pa·s; L is the liquid holdup, dimensionless; and Ag and AL are the ow areas occupied by 2 gas and liquid respectively, m . For turbulent ow (Reynolds number is usually less than 105 due to the high viscosity of mud), the friction factor is: Obtaining an analytical solution is generally not possible for most practical problems in two-phase flow. Numerical methods based on finite-difference concepts provide an alternate and powerful solution approach. The three equations, (3), (4) and (5) have been solved using the nite difference method with computational cells shown in Fig. 13, the semi-implicit pattern of which is as follows (Xu, 2002) : n 1 ) L i n ) L i 1/2 (vL )in 11/2 t (1 g )in )n 1 g i t ( n ) L i z (1 n ) L i 1/2 (vL )in 11/2 0 n n 1 g )i 1/2 (vg )i 1/2 n n 1 g )i 1/2 (vg )i 1/2 ( 0 zi 1 2 3 i-1 i+1 N-2 N-1 N 4.3 Gas unloading and accumulation at the wellhead At the wellhead, gas is released from the top when the needle valve is open. While in SCP buildup, gas accumulates at the top with the closed needle valve. Thus, two different upper boundary conditions are considered. Gas or gas-liquid flowing through the needle valve to the atmosphere can be considered as single-phase gas or multiphase flow through a choke (Perkins, 1993) . During bleed-down, gas usually ows at choked velocity and its ow rate is easy to record. If the gas ow rate cannot be measured directly, it can be computed from the following iterative procedure. a) Initial guess that the critical pressure ratio yc*=0.5, then calculate the pressure ratio y=p2/p1, where p2 is the downstream pressure (atmospheric pressure), and p1 is the upstream pressure. b) If y<yc*, critical ow exists. Calculate the gas ow rate using the following equation (Gilbert, 1954) . qgm 738.9 p1dc1h.89 1.83 qLm0.454 where dch is the diameter of the choke, m; and qLm is the liquid ow rate (m 3/s) which can be calculated by the volume of liquid collected and the bleed-down time recorded in SCP diagnostic tests. If not, sub-critical ow exists. The gas ow rate is: qgm 0.02 2 4 dch p1 p2 m1 qLm where m1 is the mixture density at p1, kg/m3. c) Calculate the gas/liquid ratio R1 (Ashford and Pierce, 1975) and yc, which is obtained from the following equation. where is the ratio of specic heats, dimensionless. Then set yc*= yc. d) Repeat step b) to c) until there is no change in the ow condition. The gas chamber in the wellhead is treated separately from all other cells, which are completely filled with gas. When the needle valve is closed, it does not lose gas to any other cell but receives gas from the cell immediately below it. The volume of this gas chamber changes with time, at n+1 time step, which is related to the n-th step volume (Xu, 2002) , in the following manner: where N is the number of equal-size cells in the well; Vwh is the volume of gas chamber, m3; Vg and VL are the volumes of gas and liquid respectively, m3; and qgm is the gas flow rate from the choke, m3/s. The second term on the right side of the equation is the volume of gas accumulated in the remaining mud column. The following term is the volume increase caused by increased liquid column pressure during this time step. The second to last term is the volume reduction due to gas ow from the cement below the liquid column. The last term indicates the increased volume caused by gas ow out from the annulus during the bleed-down, which is zero during casing pressure buildup. The wellhead pressure at n+1 time step is: pwnh1 pwnh n'n 1Vwnh Z n 1 n'nVwnh 1Z n where pwh is the wellhead pressure, MPa; Z is gas-law deviation factor, and n is the moles of gas. Once the volume of the gas chamber is estimated by Eq. (24), the wellhead pressure can be calculated from Eq. (25). This step, in turn, allows calculation of interface pressure from the wellhead pressure from Eq. (20). 4.4 Coupled gas ows in cement and mud The most important problem for coupling gas flow in the cement and the mud is calculating the interface pressure. The calculation procedure is iterative, which involves a simultaneous solution of the mass and momentum equations for pressure and velocity in all cells in the mud column except for the gas chamber. The coupling procedure is shown in Fig. 14 (Xu and (24) (25) 244.5 mm intermediate casing was bled for 12 minutes before the needle valve was closed and the following casing buildup measurement lasted 24 hours. As shown from Fig. 16, the calculation result coincides well with the actual data, which presents a long bleed-down and normal buildup pattern. 6 Conclusions Five patterns of SCP bleed-down and buildup were summarized from the field data. The possible mechanisms leading to SCP have been analyzed, which include thermal expansion, tubular mechanical failures, and gas migration. Then possible reasons for SCP in different annulus have also been indicated. On this basis, a coupled mathematical model of gas migration in a cemented annulus with a mud column has been developed based on annular percolation and gasliquid two-phase flow theories. Finally, the mathematical model was verified with selected data from one actual gas well. Acknowledgments Research work was co-financed by the China National Natural Science Foundation and Shanghai Baosteel Group Corporation (No. 51074135), Program for New Century Excellent Talents in University (No. NCET-08-0907) and Jilin Oilfield Company Project (No. JS10-W-14-JZ-32-51). Without their support, this work would not have been possible. Anders J , Rossberg S , Dube H , et al. Well integrity operations at Prudhoe Bay, Alaska . Paper SPE 102524 presented at the 2006 SPE Annual Technical Conference and Exhibition , 24 - 27 September, 2006 , San Antonio, Texas Ashford F E and Pierce P E. Determining multiphase pressure drops and ow capacities in downhole safety valves . 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Hongjun Zhu, Yuanhua Lin, Dezhi Zeng, Deping Zhang, Feng Wang. Calculation analysis of sustained casing pressure in gas wells, Petroleum Science, 2012, 66-74, DOI: 10.1007/s12182-012-0184-y