Abstract

In this paper, we illustrate an approach to underbalanced drilling (UBD) candidate selection based on damage assessment and characterization using dynamic reservoir simulations. A 2-phase, 2-dimensional transient reservoir simulator (eRes UBD) using a control volume finite element approach is used to reflect the reservoir conditions. Because of its unstructured grid formulation, the simulator is ideally suited for investigating near-wellbore phenomena, such as drilling-induced invasive damage. In addition, the simulator is augmented to capture dynamic formation exposure, thus enabling drill-ahead capability. Overbalanced drilling conditions are mimicked using the drill-ahead capability of the simulator by specifying an arbitrary depth-vs-time profile, and the resulting formation fluid distribution is investigated, with specific attention to near-wellbore saturation. The impact of invasive damage on productivity is then examined by putting the well into production. Additionally, the extent of damage, flowback period and return permeability are characterized through perturbing critical reservoir and fluid variables. Representative sector simulation runs exhibit the longer term benefits of UBD as compared to conventional counterparts. The methodology described in the paper can ultimately guide the screening and selection of UBD candidates, and is an important part of value-based selection of UBD candidates.

Introduction

The impact of damage and the need for its mitigation, remediation and/or elimination have received considerable attention in the literature as well as in drilling and production practice. Underbalanced Drilling (UBD) has emerged in recent times as an important technique for the elimination of the most pervasive form of damage- that caused by drilling overbalanced. Drilling induced formation damage can take many forms, some of which are:

  1. Invasive drilling fluid causing phase blockage thereby attenuating relative mobility of hydrocarbon phase

  2. Solids invasion occluding pore throats of the exposed formation thereby reducing the permeability in the near-wellbore region of a well. Solids come from drilling, muds or from crushing of near-wellbore rock.

  3. Uneven flow through the entire length of the horizontal well because of one or both of the aforementioned reasons

  4. Overbalanced drilling (OBD) can result in stress alteration and be the cause for unstable wellbore and permanent degradation of flow behavior

  5. Influence of the above effects may also cause unnecessary fracture initiation and propagation thereby resulting in preferential flow conduit for displacing fluids.

For reservoir applications of UBD, it is important to assess the type of formation damage and the possible effect of UBD on its mitigation. Thus, estimation and quantification of conventional overbalanced drilling induced damage is of value in UBD candidate screening.

In this work, we introduce the concept of using dynamic simulations in quantifying drilling damage, and the comparative influence of UBD on productivity when such damage is mitigated. Recent work on formation damage caused by overbalanced drilling is reviewed. A 2-phase, 2-D transient reservoir simulator has been modified to enable consideration of near-wellbore damage (through the use of unstructured grid formulation) as well as drilling ahead to mimic overbalanced drilling. Using these two features, the effect of invasive damage on productivity is illustrated for an example. Coupled with core-analysis based assessments, the methodology described here can provide a quantitative basis for the selection of UBD candidates.

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