A New Method for Vertical Leak Detection in Low to Moderate Permeability Flooded Reservoirs

Abstract

Summary Rising commodity prices have resulted in an increase in secondary recovery projects that are associated with lower permeability reservoirs. These processes often inject fluids above the parting pressure for the duration of the life of the flood, and pressurization takes place as fluids leak off from the exposed surface area of the induced fracture into the surrounding moderate or low-permeability reservoir matrix. The primary difference between these situations and conventional flooding is that matrix fluid migration is initiating off an induced fracturing plane, as opposed to point-source initiation. During the early history of a flood, mass-balance issues are often ignored, as average reservoir pressures are typically low, and injection rates can dramatically exceed production. As pressures rise, some localized portions of the reservoir not only can exceed virgin static pressures, but volumes near existing induced fractures can approach or equal the pressure in those fractures. When this occurs, vertical fracturing initiates within the reservoir, and can migrate either above or below the reservoir, depending upon the location of the closest lower stress interval. A new method to detect the location of vertical "leakage" has been developed. This method uses existing surface tiltmeter (STM) technologies and a new surface deformation calculation regime to zero in on volumetric changes that occur above or below the limits of the given reservoir. This method uses measured surface deformation from observed tilt to extrapolate the volumetric change at different specified depths. With a number of constraints and using a linear geophysical model based on poroelastic equations, an inversion routine is used to find the reservoir compaction or expansion at different depths. When volumetric change best fits the measured deformation data (or tilt) at the surface, the depth and aerial location is then correlated to the area near a specific well or wells.

Numerical Modeling and Field Monitoring of Overburden Response during Geologic Sequestration

“…For example, in hydraulic fracturing, these instruments deliver large signals as the injected fluid migrates towards the ground surface. In another example, during steam injection studies, these high-precision deformation monitoring tools have proven to provide constant updates on fluid movements and notify warnings of any fluid intrusion into inadvertent regions (Kramn et al , 2005;Walser et al, 2009). In addition to surface tiltmeters, downhole tiltmeters have also been used to determine reservoir-level strains.…”

“…In some cases, de-convolution of overburden may also be required to interpret reservoir level strains from the ground deformation results Walser et al, 2008;Walser et al, 2009). The de-convolution process requires surface displacements to be predicted and compared with measurements Walser et al, 2008;Walser et al, 2009).…”

“…In some cases, de-convolution of overburden may also be required to interpret reservoir level strains from the ground deformation results Walser et al, 2008;Walser et al, 2009). The de-convolution process requires surface displacements to be predicted and compared with measurements Walser et al, 2008;Walser et al, 2009). As discussed earlier, the prediction of surface displacements is possible with any one of the displacement models proposed by Okada (1985 and or Green and Sneddon (1950) or Segall (1985) or Vasco (1998).…”

“…As discussed earlier, the prediction of surface displacements is possible with any one of the displacement models proposed by Okada (1985 and or Green and Sneddon (1950) or Segall (1985) or Vasco (1998). If the predictions are not good, the assumptions are modified and the inversion process is continued until a best fit is attained or predicted displacements match reasonably well with measured surface displacements (Vasco et al, 1998;Davis et al, 2005;Walser et al, 2008;Walser et al, 2009). A Monte Carlo approach or brute force error mapping approach can also be followed to perform sensitivity studies to determine the uncertainties in the model (Davis et al, 2005).…”

“…In such cases, poroelastic models demonstrated by Segall (1985) or Vasco (1998) are suggested useful in predicting ground deformations (Davis et al, 2005). Surface displacements caused due to changes in the pore fluid volume are calculated based on the Equation (43) as explained in many other research papers (Vasco et al, 1998;Davis et al, 2005;Du et al, 2007;Walser et al, 2008;Walser et al, 2009). 4.3Surface tilts are computed by taking the gradient of vertical displacements and can be presented as shown in Equation (4.4) (Vasco et al, 1998;Davis et al, 2005;Du et al, 2007;Walser et al, 2008;Walser et al, 2009): 4.4where, (Okada,1992;Segall, 1985).…”