Estimation of Uniaxial Compaction Coefficient from GPS Measured Subsidence by Dr.PARUL R. Patel Senior Associate Professor Nirma University, Ahmedabad parul.patel@nirmauni.ac.in

Organization of Presentation Introduction Objectives of the Study GPS Field Data Collection & Processing Methodology Analysis of Results Estimation of Uniaxial Compaction Coefficient Conclusions

Man-made Subsidence Occurs mainly due to Land Subsidence Land subsidence is a gradual settling or sudden sinking of the earth's surface with or without horizontal movement Man-made Subsidence Occurs mainly due to Mining activities Underground construction activities Extraction of natural resources like water/gas/oil

Mechanism of Land Subsidence due to Gas Extraction Weight of overlying sediments partially supported by soil matrix and partially by gas/oil present in pores. Pressure p declines, and overburden load transferred to soil matrix. Compaction takes place if soil is compressible Bowl-shaped depression appears with large displacement at centre.

Effects of Land Subsidence Even Settlement rarely causes any problems but uneven settlement creates problems Changes in Elevation and Slope of Canals, Drains, Pipe lines etc Damage to Infrastructure Failure of Well Casings Water Intrusion in Coastal Areas Reduction in Permeability, Loss of Hydrocarbon productivity Fault Reactivation Resulting in Seismic Activity Development of Fissures and Sinkholes

Objectives of the Study Use of GPS technique to monitor subsidence using a case study near Olpad Region, Surat, Gujarat To identify and study effects of parameters responsible for land subsidence, like, pressure depletion, gas extraction, water level, Uniaxial Compaction Coefficient, Reservoir dimensions Calculation of Uniaxial Compaction Coefficient

GPS used for Monitoring Subsidence Ekofisk Oil Field, 1991-93, Single Frequency Novtel, Novtel Software used Groningen Gas Field, 1992-1993, Dual Frequency, Bernese Software Coachella Valley, California, 1996-98, Dual Frequency, GP Survey Software Rafsanjan Plain, Iran, 1998-1999, Leika, SKIPro Software Ojiya City, Japan, 1996-98 , Jakarta, Indonesia, 1997-2001, Dual Frequency, Bernese Software

Reference Network & Monitoring Stations with Reservoir Boundary Deformation Stations with Reference Stations Locations of IGS Stations Deformation Stations and Presumed Reservoir Boundary Details of the Study Area

GPS Monument at Monitoring Station C/S of GPS Monument 4000 ssi Receiver with Choke Ring Antenna

Processing Parameters and Methodology Processing Mode Software : Post Processing Mode : Bernese V4.2 Types of orbit : Precise Ephemeris (SP3) IGS stations : LHAS,BAHR,IISC(ITRF-2000) Ionospheric Correction : Differential Correction and combining L1 & L2 frequencies Tropospheric correction PDOP Angle 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.

Average Effective Subsidence for May Campaigns - May 05 May 06 March 07 Av. Effective Subsidence within Reservoir Boundary (mm) 29 ± 5 25 ± 5 32 ± 5 86 ± 5 Rate of Subsidence 30 mm/year within Reservoir Boundary

Study of Reservoir Pressure and its behaviour May 2004 June 2007 Max. Pressure: 2290 kN/m2 at NSA4 Min. Pressure : 2034 kN/m2 at NSA2 Av. pressure of the reservoir: 2105 kN /m2 Max Pressure: 1336 kN/m2 at NSA2 Min. Pressure: 989 kN/m2 at NSA1 Av. pressure of the reservoir: 1098 kN/m2 Maximum Pressure Depleted on north side More Numbers of Gas Wells Located on North side Maximum Subsidence observed on North side

Relation Between Gas Extraction and Pressure Depletion Months Cumulative Gas Extraction (m3) Average Gas Pressure (kN/m2) May 2004 5140820 2105 December 2004 51968697 2004 May 2005 92888305 1802 December 2005 146809691 1615 May 2006 186793054 1471 December 2006 245242665 1278 June 2007 291922138 1098 Linear relation Between Gas Extraction & Average Pressure Pressure is Depleting Continuously with increase in Cumulative Gas Extraction

Relation between Cumulative Effective Subsidence & Cumulative Gas Extraction Campaigns Cumulative Gas Extraction (m3) from four gas wells Average Effective Subsidence (∆s) for four stations May.04 - Oct.04 3.54E+07 -11 May.04 - Feb.05 6.86E+07 2 May.04 - May.05 9.29E+07 -38 May.04 - Octo.05 1.3E+08 -67 May.04 - Jan.06 1.63E+08 -60 May.04 - Mar.06 1.71E+08 -42 May.04 - May.06 1.87E+08 -66 May.04 - Oct. 06 2.28E+08 -48 May.04 - Jan.07 2.54E+08 -71 May.04 - Mar.07 2.7E+08 -91 Linear Relationship is observed to be Suitable with Regression Analysis Correlation Coefficient is Found to be High 0.83 Gas Extraction is the major cause of Subsidence over the Study area

Relation between Water Level and Subsidence Seasonal Change Observed in Water Level and in Ellipsoidal Height Change in Water Level (Four Wells) Change in Ellipsoidal Height (Four Wells) No permanent water depletion observed Net reduction in the average Ellipsoidal Height

Estimation of Uniaxial Compaction Coefficient (Cm) Laboratory Measured Cm Cm = 2.7E-05 m2/kN Where, Modulus of Elasticity (E) =2.5E04 kN/m2 (Laboratory Measured) And Poisson’s Ratio (υ) = 0.33 Subsidence = Cm H Δp Subsidence = 953 mm For (Δp = 905 kN/m2, H=39 m) Subsidence Very High Compared to Measured Subsidence (86 mm) Laboratory Measured Cm Value usually overestimated

Average Cm for Four Wells Cm values Ranging from 1.27E-06 m2/kN to 3.70E-06 m2/kN Average Cm Value is 1.84E-06 m2/kN ∆s = (-1.84E-06) (∆p * H) R = 0.73 Cm = 1.84E-06 m2/kN For Barbara Field, Field Measured Cm Values were 2 x 10-7 to 5 x 10-7 m2/kN Laboratory Measured Cm values ranging from 1 x 10-6 to 5 x 10-5 m2/kN Laboratory Measured Cm is more than ten times to Field Measured Cm with GPS

Subsidence Prediction based on Cm Calculated using Nucleus of Strain Method (Geertsma, 1978) (Assuming Reservoir Compaction ≠ Subsidence) Subsidence at the Centre of the Reservoir Value of ‘A’ depends on two Dimensionless ratios η=D/R and ρ = r/R D 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

Conclusions Effective subsidence over the study area was 86 mm during February 2004 to March 2007 within reservoir boundary Subsidence 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 = 0.534 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.

Thank You

Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 years Reservoir Thickness = 30 m Subsidence measured with GPS May, 2004 to March 2007 Pressure Depletion = 1000 kN/m2 Pressure Depletion = 1200 kN/m2 Actual pressure depletion of 905 kN/m2 (Darcy, 2006 ) Linear Compressibility Non linear Full –Field Reservoir model and realistic depletion strategy, FEM used Based on Linear Compressibility 809 mm 720 mm 610 mm 733 mm 86 mm

Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 years Based on Linear Compressibility, H = 30 m and actual pressure depletion of 905 kN/m2 (Darcy, 2006 ) Subsidence measured with GPS May, 2004 to March 2007 Pressure Depletion = 1000 kN/m2 Pressure Depletion = 1200 kN/m2 Linear Compressibility Non linear Full –Field Reservoir model and realistic depletion strategy, FEM used 809 mm 720 mm 610 mm 733 mm 86 mm

Subsidence Predicted by Taurus (2003) Reservoir Thickness = 30 m; Duration 17 years Reservoir Thickness = 30 m Subsidence measured with GPS May, 2004 to March 2007 Pressure Depletion = 1000 kN/m2 Pressure Depletion = 1200 kN/m2 Actual pressure depletion of 905 kN/m2 (Darcy, 2006 ) Linear Compressibility Non linear Full –Field Reservoir model and realistic depletion strategy, FEM used Based on Linear Compressibility 809 mm 720 mm 610 mm 733 mm 86 mm