Borehole stability in deplated fields

Abstract

Drilling problems are increasing in depleted fields. Common drilling problems in depleted fields are borehole collapse, differential sticking, and lost circulation in depleted sands. To suggest the solution schemes to the problems we have to first find out the cause of problems hence understand the geomechanical criteria that form the basis for the borehole stability.

 Chapter 2. introduction in formation mechanics introduces basic criteria for borehole stability that are fundamental for borehole stability analysis. Sub-chapter 2.3 shear failure criterion shows how important Mohr Coulomb criterion or Mohr Coulomb circle are for determining borehole stability with respect to shear faillure. As concluded in sub-chapter 2.5, mud weight window, the minimum horizontal stress of sands is determining for the upper bound of the mud weight window in fractured formations including alternating shale and depleted sand sections. Collapse pressures of shale and pore pressure of shale are determining for the lower bound of the mud weight window. If the formation is not fractured, the fracture initiation pressure of sands will be determining for the upper limit of the mud weight window.

Differential sticking is a result of using a higher mud weight than allowed in sands, and is a common problem in depleted sand zones. There exist some factors that are determining for whether differential sticking will occur or not, for instance the contact area the differential pressure is acting on, the magnitude of overbalance and the time the hole has been exposed to overbalance. Moreover, the well trajectory is determining for the probability of differential sticking.

Differential sticking is a result of using a higher mud weight than allowed in sands, and is a common problem in depleted sand zones. There exist some factors that are determining for whether differential sticking will occur or not, for instance the contact area the differential pressure is acting on, the magnitude of overbalance and the time the hole has been exposed to overbalance. Moreover, the well trajectory is determining for the probability of differential sticking.

Differential sticking and borehole collapse can also cause stuck pipe. Sub-chapter 2.6.9 discusses the method for diagnosing the cause of stuck pipe. This sub-chapter concludes with that analyzing observations done during drilling about circulation parameters and about other drilling parameters can help us identifying the cause of stuck pipe.

When drilling is going in alternating shale and depleted sand sections, risk of tensile failure in sands section also exists. Tensile failure in sands can result in lost circulation. As explained in sub-chapter 3.6.2, losses, lost circulation will occur in sands if the minimum horizontal stress in sands and fracture initiation pressure of sands, pf, are exceeded. Losses result in delay in drilling operations hence increase in NPT. Moreover, a temporary pressure drop in the well can cause kicks, inflow of formation fluids into the borehole.

Many solutions schemes can be applied to prevent the mentioned borehole instabilities and the main conclusion of this thesis is that borehole instabilities in alternating shale and depleted sands can be prevented by applying measures that can result in widening of the MWW.