Wellbore Cooling as a Means To Permanently Increase Fracture Gradient

Abstract

Abstract In recent years, several techniques have been proposed to increase the fracture gradient by inducing changes in the near wellbore region (Alberty and McLean, 2004; Sweatman, et al., 2004; Benaissa et al., 2005). This process, often called "wellbore strengthening", has most recently been implemented by adding specially designed proppant material to the mud before raising its pressure above the fracture gradient. The goal was to induce short tensile fractures in the vicinity of the wellbore wall which are prevented from propagating; thus, creating a "stress cage". However, this method has often proven ineffective in low permeability formations where mainly uncontrolled fracture propagation occurs. The purpose of this paper is to propose and evaluate the use of wellbore cooling, in combination with more classical stengthening processes, to permanently increase the fracture gradient without the risk of circulation losses inherent in the "stress cage" method, as it is currently applied. This approach involves lowering the temperature of the drilling mud; thus, reducing the hoop stress at the borehole wall and then ‘setting’ the stress cage in the standard manner. Tensile cracks can then be induced at significant lower mud weights. Given the typical thermal conductivity properties of rocks, the tensile stresses induced by cooling (and consequently, the created fractures) will tend to be confined to the near wellbore region. This work presents an evaluation of the effect cooling has on the stress profile of a "solid" material and compares it with a fully coupled thermoporoelastic solution. The results of such analyses may then be used to design a field application to test this novel idea. Introduction As the demand for hydrocarbon resources grows, drilling is increasingly taking place in demanding and hazardous environments. Current oil and gas plays are located in basins where often the drilling of horizons with low fracture gradient is involved. Such formations present the drilling engineer with a scenario where the operational mud weight window to be used is rather limited or sometimes, non-existant. These formations tend to be weak; thus, requiring high mud weights in order to avoid borehole collapse, while simultaneously exhibiting low fracture gradients. A similar situation is found in depleted sands; where such formations show lower fracture gradient, whereas adyacent shales need to be controlled with high mud density due to their high pore pressure. In order to overcome this constraint, different methods for increasing the fracture gradient of underground formations have been put forward over the years. Very recently, Benaissa et al. (2005) proposed a method that increases the fracture gradient by using particles to seal off the pores in the formation at the wellbore face; thus, creating an effective non-porous region immediately behind the borehole (i.e. a "sleeve" region at the wall). The final result being a large increment in the tensile strength of the rock. However, one of the most successful techniques for increasing the fracture gradient of a subsurface horizon is the application of the "stress cage" concept. In this method, the tangential stress around the wellbore is increased by inducing and propping open a controlled fracture at the borehole wall (Alberty and McLean, 2004). This technique, albeit very efficient in permeable formations, has however proven rather ineffective when applied to low permeability rocks. This is the case of "inverted" stress profiles, where the fracture gradient in the shale is lower than in adjacent sands. This paper proposes a new procedure for creating such stress cage in low-permeability formations (e.g. shaly sands). In this novel method, changes in temperature are induced in the formation to be treated, before "setting" the stress cage. Drilling fluid is used to cool down the formation in order to reduce the tangential stress at the borehole wall. The magnitude of this temperature change is determined by the required increment in fracture resistance, which also establishes the opening of the fractures in the stress cage. Subsequently, the stress cage is set up following normal procedures.