Periodical meeting CO2Monitor. Leakage characterization at the Sleipner injection site

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1 Periodical meeting CO2Monitor Leakage characterization at the Sleipner injection site Stefano Picotti, Davide Gei, Jose Carcione

2 Objective Modelling of the Sleipner overburden to study the sensitivity of the seismic method to leackage detection Picotti S., Gei G., Carcione J.M., 2013: Leackage detection with seismic methods at the Sleipner field: a synthetic study. In progress Petrophysical model Saturation model (leakages) Seismic modelling Sleipner injection site (North Sea) The Sleipner storage site is located in the Norwegian sector of the North Sea. This is the first commercial scale CCS project, which began in 1996 with the injection of the CO2 separated from the extracted natural gas into a deep saline aquifer contained in the Utsira Sand formation, at approximately 1 km depth below the sea bottom. Since 1996 about 1 Mt CO2 per year has been injected.

3 Utsira Formation Seismic section Overburden Main Characteristics Thickness: m Average Depth: 950 m Geology: highly permeable unconsolidated sand with mudstone intra-layers 1 meter thick. Average permeability: 3Darcy Average porosity: 37% T=29 degc; P=80bars; 3% CH4 saturation (the CO2 is almost pure)

4 Well-logs (15/9-13 well) Geological model Sea water Overburden 1 Overburden 2 Overburden 3 Drop Leakage Caprock 1 Caprock 2 Caprock 3 Caprock 4 Reservoir (Utsira) Formation below Utsira

5 Real seismic line Sea water Overburden 1 Overburden 2 Overburden 3 Caprock 1 Caprock 2 Caprock 3 Caprock 4 Reservoir (Utsira) Formation below Utsira 5

6 Geological model Sea water Overburden 1 Overburden 2 Overburden 3 Caprock 1 Caprock 2 Caprock 3 Caprock 4 Reservoir (Utsira) Formation below Utsira

7 Baseline Real seismic section Synthetic seismic section Good match between the Synthetic seismic section and the Real seismic section

8 Overburden Leakage Sea water Overburden 1 Overburden 2 Overburden 3 Leakage Caprock 1 Caprock 2 Caprock 3 Caprock 4 Reservoir (Utsira) Formation below Utsira

9 Construction of the poro-viscoelasctic model FLUIDS properties: Brine or Oil Density, Bulk Modulus and Viscosity Equations of Batzle-Wang GAS PROPERTIES: CO2 and CH4 Density, Bulk Modulus and Viscosity Equation of state of Peng-Robinson + Lohrenz-Bray-Clark theory Mixing rules Mixed gas properties: CO2 + CH4 Poro-viscoelastic model POROUS MEDIUM properties: Density, Bulk Modulus, Porosity and Permeability Experimental measurements WAVE SIMULATIONS

10 Brine and CO2 properties versus depth A very small part of the gases can dissolve in Brine. Almost all the gas (CO2 and/ or CH4) remains as free gas. The properties of the gas are computed using the Peng-Robinson EoS. The properties of Brine are computed using the equations of Batzle and Wang (1992). Salinity Utsira = ppm T at the sea bottom = 4.8ºC Geothermal gradient= 31.7 ºC/km Gas saturation in the leakage: 10% L: Leakage 500m depth CO2 gas R: Reservoir 900m depth CO2 supercritic

11 TWT (s) Calculus of synthetic seismograms to compare with the real ones or for a preliminary evaluation of the reliability of a survey. However. Due to the extremely fine meshes needed to represent these type of media, numerical simulations using Biot s equations of motion at the macroscale is very expensive or even not feasible.

12 ALTERANTIVE APPROACH. Determine equivalent complex P-wave and shear moduli solving at the mesoscale (and for a finite number of frequencies) boundary value problems representing oscillatory compressibility and shear tests on a representative volume of bulk material containing multiscale stochastic heterogeneities. The oscillations of these samples are assumed to obey Biot s equations of motion, discretized using a finite element procedure. The (complex) oscillatory volume change suffered by the sample, yields the equivalent undrained complex P-wave modulus. The change in shape allow to recover its equivalent undrained complex shear modulus.

13 Representative rock samples The exact spatial distribution of these heterogeneities is in general unknown. Instead, they will be assumed to be stochastic functions characterized by their statistical properties. Leakages Fluid mix saturation: 10% Reservoir Fluid mix saturation: 56% The patchy saturated regions were determined using stochastic fractal fields based on the Von Karman selfsimilar correlation functions. Side length 50 cm, fractal dimension D=2.5, correlation length a = 5cm. White zones: pure fluid mixture saturation, black zones: pure brine saturation.

14 Results of the Oscillatory tests P-wave Velocity P-wave Q-factor For a given realization of the stochastic parameters, the computed displacements allow to determine equivalent complex P-wave and shear moduli as functions of frequency, representing the behavior of the heterogeneous material at the macroscale. We calculate the means and variance of Vp and 1/Qp averaging over many patchy realizations of statistical parameters (Montecarlo).

15 Zener best fit We define an equivalent viscoelastic model fitting the behaviour of the computed complex moduli in the desired range of frequencies using a Zener model. This model is used at the macroscale to perform numerical simulations of wave propagation phenomena in the time domain. work in progress...