ABSTRACT

This paper presents finite-element simulation results for hydraulic fracture''s initiation, propagation and sealing in the near wellbore region. The main objective of these simulations was to investigate the hypothesis of wellbore hoop stress increases when fractures are wedged and/or sealed during lost circulation control. To address this objective three-dimensional poro-elastic models were solved with finite-element simulations. Three analytical solutions were also reviewed to investigate other mechanisms of increasing fracture propagation pressure by fracture sealing, these models then were used to predict a new fracture gradient and compare it to the numerical simulation results. The situation was also investigated from a fracture mechanics perspective where analogous bridging and toughening mechanisms were applied to increase fracture resistance in other materials. Cohesive zone modeling was used as the primary methodology for simulating fractures, this enabled us to assign individual criteria for fracture initiation and propagation in each model. Our results demonstrate that fracture wedging is not able to increase wellbore hoop stress more than its ideal state where no fracture exists, however this will help to restore part or all of the wellbore hoop stress lost during fracture propagation which can act as secondary mechanism for increasing wellbore fracture gradient. The alternative mechanism can be explained the existence of a strong barrier and/or non-invading zone inside the fracture which prevents further communication between wellbore and propagated fracture.

Introduction

The complexity of modem oil and gas exploitation requires a better understanding of geomechanical behavior of rocks at depth. This ranges from understanding large scale geology features like fault behavior and salt diapirs to near wellbore issues such as wellbore stability concerns. Most important among these are tensile failure causing mud losses and borehole collapse hindering further drilling. The magnitude of this effect is increased when drilling in deep offshore basins or highly deviated wellbores. One major concern in these situations is reduction of the safe mud weight window between pore and fracture pressure which is a crucial factor in well design. Total overburden density decreases in deep offshore basins since the uppermost interval is water, which has considerably less density than rock, and this reduces formation fracture resistance.

The routine practice to stop lost circulation problem is to add loss circulation materials (LCMs) to the mud system. Since this process is often based on trial and error, the results are not consistent and the mechanisms behind various means are not fully understood. Several analytical solutions exist for modeling fractures in rocks, however, since fracturing in rocks is a non-linear process, which is also associated with complex boundary conditions, these models requires several simplifications which reduce their applicability. For this reason numerical modeling, especially Finite Element Analysis (FEA), has been extensively used to model fractures. The main procedure followed in this paper was to model fractures as a continuous multistage process with poro-elastic material models. The methodology used for fracture simulations was based on cohesive zone modeling. Fracture experiment results indicate that this approach best describes fracture behavior in quasibrittle materials such as rocks which show subcritical fracture behavior and non-linear stressstrain response.

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