Abstract
Solids production is relatively uncommon in tight reservoirs, but it has been identified as a common problem in the fractured, superdeep, hot tight gas reservoirs in the Tarim Basin of China (TVD is 21,000+ ft., temperature is 320+ °F, closure stress is 20,000+ psi, formation pressure is 14,500+ psi, drawdown pressure is very low, etc.). We are investigating the fundamental mechanisms governing solids production in such reservoirs, and exploring for potential solutions.
It is necessary to recognize the differences between failure mechanisms and sand production potential in unconsolidated/weakly consolidated sands and indurated, low permeability sandstones. We have observed several phenomena during and after fracturing in this high-temperature, low-permeability reservoir. These are: (1) there is a substantial difference between the formation temperature and bottomhole fluid treating temperature during fracturing, about 230+ °F; (2) the fracturing fluid has an high salinity (40% NaNO3 was used to the fracturing fluid to increase weight/hydrostatic head in order to reduce surface treating pressure) and there has been a long interaction time due to partial flowback; (3) there is an extremely high gas production rate; and (4) on occasion, there has been aggressive and frequent bean-up or shut-in. After recognizing these complicating considerations, we analyzed their impact on rock integrity, and on the resistance to solids production (both proppant and formation). We are conducting an in-depth analysis of field observations of sanding. With this effort, we are attempting to develop solutions to mitigate or prevent sand production for future wells.
There is a history of significant solids production. The large temperature difference between the static formation temperature and the fracturing fluid temperature incurs extreme thermal stress near the wellbore, and to a lesser degree, along the fracture. These thermoelastic/plastic effects may incur damage near the wellbore, through the completion and along the fracture surface. In addition, failure by shear or extension might be induced in the intact tight rock with extended contact with the high salinity base fracturing fluid. Hydrodynamic drag and significant energy expenditure due to high gas rates, non-Darcy losses, and in-situ erosion are anticipated. Some authors have coined a term, "C-Factor," to quantify this rate sensitive behavior. The "C-Factor" embodies the second order rate dependency of kinetic energy expenditures. With large velocities, kinetic energy expenditure is observed to be extremely high, and may be a root cause for damage to perforation tunnels and intact rock. In order to prevent solids production in the future, we may need to consider controlling salinity, production rate, the temperature of fracturing fluid, bean-up and shut-in frequency, along with standard geomechanical controls such as borehole trajectory.
This paper speculates on some of the fundamental mechanisms governing solids production in extremely deep, high temperature, low permeability gas reservoirs; specifically the Tarim Basin.
