Presentation on theme: "by Dr.PARUL R. Patel Senior Associate Professor"— Presentation transcript:
1 Estimation of Uniaxial Compaction Coefficient from GPS Measured Subsidence byDr.PARUL R. PatelSenior Associate ProfessorNirma University, Ahmedabad
2 Organization of Presentation IntroductionObjectives of the StudyGPS Field Data Collection & Processing MethodologyAnalysis of ResultsEstimation of Uniaxial Compaction CoefficientConclusions
3 Man-made Subsidence Occurs mainly due to Land SubsidenceLand subsidence is a gradual settling or sudden sinking of the earth's surface with or without horizontal movementMan-made Subsidence Occurs mainly due toMining activitiesUnderground construction activitiesExtraction of natural resources like water/gas/oil
4 Mechanism of Land Subsidence due to Gas Extraction Weight of overlyingsediments partiallysupported by soil matrix and partially by gas/oil present in pores.Pressure p declines, and overburden loadtransferred to soil matrix.Compaction takes place if soil is compressibleBowl-shaped depressionappears with largedisplacement at centre.
5 Effects of Land Subsidence Even Settlement rarely causes any problems but uneven settlement creates problemsChanges in Elevation and Slope of Canals, Drains, Pipe lines etcDamage to InfrastructureFailure of Well CasingsWater Intrusion in Coastal AreasReduction in Permeability, Loss of HydrocarbonproductivityFault Reactivation Resulting in Seismic ActivityDevelopment of Fissures and Sinkholes
6 Objectives of the Study Use of GPS technique to monitor subsidence using a case study near Olpad Region, Surat, GujaratTo identify and study effects of parameters responsible for land subsidence, like, pressure depletion, gas extraction, water level, Uniaxial Compaction Coefficient, Reservoir dimensionsCalculation of Uniaxial Compaction Coefficient
7 GPS used for Monitoring Subsidence Ekofisk Oil Field, ,Single Frequency Novtel, Novtel Software usedGroningen Gas Field, ,Dual Frequency, Bernese SoftwareCoachella Valley, California, , Dual Frequency, GP Survey SoftwareRafsanjan Plain, Iran, ,Leika, SKIPro SoftwareOjiya City, Japan, ,Jakarta, Indonesia, , Dual Frequency, Bernese Software
8 Reference Network & Monitoring Stations with Reservoir Boundary Deformation Stations with Reference StationsLocations of IGS StationsDeformation Stations and Presumed Reservoir BoundaryDetails of the Study Area
9 GPS Monument at Monitoring Station C/S of GPS Monument4000 ssi Receiver with Choke Ring Antenna
10 Processing Parameters and Methodology Processing ModeSoftware: Post Processing Mode: Bernese V4.2Types of orbit: Precise Ephemeris (SP3)IGS stations: LHAS,BAHR,IISC(ITRF-2000)Ionospheric Correction: Differential Correction and combining L1 & L2 frequenciesTropospheric correctionPDOPAngle of Elevation: SAASTAMOINEN Model (Hugentobler & Satirapod) & site specific piecewise linear method: 4 - for better Satellite Geometry (Rabbany, 2002): 15° (to avoid signals from lower satellites to reduce tropospheric error)IGS stations tightly constrained, to get coordinates of all the four reference stations and IITB permanent reference station.Coordinates of all 27-deformation stations were processed with two reference stations and IITB station by tightly constraining them.
11 Average Effective Subsidence for May Campaigns -May 05May 06March 07Av. Effective Subsidence within Reservoir Boundary (mm)29 ± 525 ± 532 ± 586 ± 5Rate of Subsidence 30 mm/year within Reservoir Boundary
12 Study of Reservoir Pressure and its behaviour May 2004June 2007Max. Pressure: kN/m2 at NSA4Min. Pressure : kN/m2 at NSA2Av. pressure of the reservoir: 2105 kN /m2Max Pressure: kN/m2 at NSA2Min. Pressure: 989 kN/m2 at NSA1Av. pressure of the reservoir: kN/m2Maximum Pressure Depleted on north sideMore Numbers of Gas Wells Located on North sideMaximum Subsidence observed on North side
13 Relation Between Gas Extraction and Pressure Depletion MonthsCumulative Gas Extraction (m3)Average Gas Pressure (kN/m2)May 20042105December 20042004May 20051802December 20051615May 20061471December 20061278June 20071098Linear relation BetweenGas Extraction & Average PressurePressure is Depleting Continuously with increase in Cumulative Gas Extraction
14 Relation between Cumulative Effective Subsidence & Cumulative Gas Extraction CampaignsCumulative Gas Extraction (m3) from four gas wellsAverage Effective Subsidence (∆s) for four stationsMay.04 - Oct.043.54E+07-11May.04 - Feb.056.86E+072May.04 - May.059.29E+07-38May.04 - Octo.051.3E+08-67May.04 - Jan.061.63E+08-60May.04 - Mar.061.71E+08-42May.04 - May.061.87E+08-66May.04 - Oct. 062.28E+08-48May.04 - Jan.072.54E+08-71May.04 - Mar.072.7E+08-91Linear Relationship is observed to beSuitable with Regression AnalysisCorrelation Coefficient is Foundto be High 0.83Gas Extraction is the major cause of Subsidence over the Study area
15 Relation between Water Level and Subsidence Seasonal Change Observed in Water Level and in Ellipsoidal HeightChange in Water Level (Four Wells)Change in Ellipsoidal Height (Four Wells)No permanent water depletion observedNet reduction in the average Ellipsoidal Height
16 Estimation of Uniaxial Compaction Coefficient (Cm) Laboratory Measured CmCm = 2.7E-05 m2/kNWhere, Modulus of Elasticity (E) =2.5E04 kN/m2(Laboratory Measured)And Poisson’s Ratio (υ) = 0.33Subsidence = Cm H ΔpSubsidence = 953 mmFor (Δp = 905 kN/m2, H=39 m)Subsidence Very High Compared to Measured Subsidence (86 mm)Laboratory Measured Cm Value usually overestimated
17 Average Cm for Four Wells Cm values Ranging from1.27E-06 m2/kNto3.70E-06 m2/kNAverage Cm Value is1.84E-06 m2/kN∆s = (-1.84E-06) (∆p * H)R = 0.73Cm = 1.84E-06 m2/kNFor Barbara Field, Field Measured Cm Values were 2 x 10-7 to 5 x 10-7 m2/kNLaboratory Measured Cm values ranging from 1 x 10-6 to 5 x 10-5 m2/kNLaboratory Measured Cm is more than ten times to Field Measured Cm with GPS
18 Subsidence Prediction based on Cm Calculated using Nucleus of Strain Method (Geertsma, 1978) (Assuming Reservoir Compaction ≠ Subsidence)Subsidence at the Centre of the ReservoirValue of ‘A’ depends on two Dimensionless ratios η=D/R and ρ = r/RD is the Depth of Burial (175 m), R is radius of Reservoir (2500 m), r is the distance of point from centre of the Reservoir (r=0)This Cm value used to predict subsidence from time to time over this reservoir
19 ConclusionsEffective subsidence over the study area was 86 mm during February 2004 to March 2007 within reservoir boundarySubsidence is directly related to the amount of gas extracted and resulting pressure. A linear relationships are observed between:Cumulative gas extraction and average reservoir pressure (R =0.99)Subsidence and gas extraction ( R = 0.83)subsidence and pressure depletion ( R = to 0.82)The average compaction coefficient Cm determined from GPS studies (assuming subsidence = compaction) is found to be 1.84E-06 m2/kN.Subsidence prediction based on field measured is more acceptable than the laboratory measured uniaxial compaction coefficient, as it is always higher by one order of magnitude than the actual measured subsidence in the field.Estimating Cm (assuming subsidence = compaction) based on GPS measured parameters and Nucleus of Strain Method is found to be 1.95E-06 m2/kN.
21 Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 yearsReservoir Thickness = 30 mSubsidencemeasured withGPSMay, 2004 toMarch 2007Pressure Depletion = 1000 kN/m2Pressure Depletion = 1200 kN/m2Actual pressure depletion of 905 kN/m2(Darcy, 2006 )LinearCompressibilityNon linearFull –FieldReservoir model and realisticdepletion strategy, FEM usedBased on Linear Compressibility809 mm720 mm610 mm733 mm86 mm
22 Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 yearsBased on Linear Compressibility, H = 30 m and actual pressure depletion of 905 kN/m2(Darcy, 2006 )Subsidencemeasured withGPSMay, 2004 toMarch 2007Pressure Depletion = 1000 kN/m2Pressure Depletion = 1200 kN/m2LinearCompressibilityNon linearFull –FieldReservoir model and realisticdepletion strategy, FEM used809 mm720 mm610 mm733 mm86 mm
23 Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 yearsReservoir Thickness = 30 mSubsidencemeasured withGPSMay, 2004 toMarch 2007Pressure Depletion = 1000 kN/m2Pressure Depletion = 1200 kN/m2Actual pressure depletion of 905 kN/m2(Darcy, 2006 )LinearCompressibilityNon linearFull –FieldReservoir model and realisticdepletion strategy, FEM usedBased on Linear Compressibility809 mm720 mm610 mm733 mm86 mm