Abstract

Although numerous sandstone and carbonate simulators have been developed during the past decade, few have been field validated. This paper addresses the field validation of a numerical simulator used for treatment design.

Five matrix acidized wells were used for validation of the simulator. In most cases the simulator was within +10% of the actual skin reduction observed in the well. The simulator calculates the pressure at the formation face and within the multiple layers along the corresponding flow rates. Diversion and mineral dissolution with the corresponding permeability changes are also calculated for sandstone and carbonates. The key to simulator validation is good well data including pre and post-treatment pressure buildup analysis, PLT data, log and/or core data, formation mineralogy and the knowledge of the damage mechanism.

Simulations indicate previously developed "rule of thumb" guidelines for mud acid volume may not yield the best results. When formation damage is shallow, as in some of the case histories the "rule of thumb" may results in the use of excessive acid, whereas, when damage is 2 to 3 feet from the wellbore higher volumes of acid are normally required. Simulations support the concept that diversion is essential and can easily be observed via the flow per layer output. This study indicates matrix treatment design is not engineered until it is simulated using valid models. Application of the validated simulator results in increased production and improved economics for the operator. A detailed description of the validation process and the supporting well data are presented.

Introduction

Sandstone matrix acidizing using mud acid formulations has been used for decades to remove siliceous formation damage. The damage can be formed during drilling, completion and/or production phases of a well, which can cause severe production decreases. Numerous papers have been written on laboratory and field studies directed at clay damage removal. These laboratory studies were conducted in support of development of acidizing simulators. However, none of the simulators were field validated. This paper will address this issue by using well documented case histories.

The numerical simulator used in this study was previously described in the literature. The simulator is 2d and is capable of acidizing and diverting fluids. Fluid fingering, acid concentrations and fluid saturation at each point radially in the formation are calculated at each time step. The model considers diversion by using cake resistance or a pseudoskin correlation. Dissolution of the mineral species is based on a change in porosity. The porosity is converted to permeability based on a modified Labrid's formula.

The three sandstone cases presented are graveled pack wells from the Gulf of Mexico. Two wells were suspected to have HEC polymer damage and the third well was suspected to have silt and clay damage. Information for each of the wells varied from detailed laboratory flow studies to porosity logs. The later is probably what most operators have for their well description. In all three cases we were able to model the pre and post skin results.

The two carbonate cases are wells from the Middle East. They represent the acids used routinely in carbonate reservoirs (HCl and emulsified HCl). Two distinct models are used in the simulator process. The first model is cable of modeling wormhole growth used for non-retarded HCl whereas the second model assumes uniform dissolution by a emulsified (retarded) acid i.e. radial flow through all pore throats.

Sandstone Acidizing Validation Case Histories Modeling. During sandstone acidizing the numerical simulator models the dissolution of formation damage and native minerals. P. 283^

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