Wormholing in Perforated Completions
- Nitika Kalia (Halliburton) | Vemuri Balakotaiah (U. of Houston)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium and Exhibition on Formation Damage Control, 10-12 February, Lafayette, Louisiana, USA
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 3.2.4 Acidising, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 2 Well Completion, 4.3.4 Scale, 5.8.7 Carbonate Reservoir, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.2.2 Perforating, 1.6.9 Coring, Fishing, 1.8 Formation Damage
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Matrix acidizing of carbonates is routinely carried out to increase hydrocarbon production rates from a reservoir. This involves pumping acid through the wellbore into the formation where it reacts with the rock and results in an increase in permeability. As a result of the dissolution reaction, different types of dissolution patterns are created. These patterns depend on the injection rate, type of fluid, rock mineralogy, temperature, flow geometry, etc. The effect of these factors on dissolution patterns has been studied extensively in the past. However, most of the reported experimental and theoretical work presents wormholing under openhole conditions. When the well completion includes perforated casing, fluid-flow through the perforations is markedly different from that in an openhole completion. The degree of success of an acid job in a perforated completion is limited by the proximity of the damage to the perforation tunnel. The wormhole pattern structure, penetration distance, and acid volume required to achieve a given permeability increase in such a completion is still not clearly understood.
In this paper, the effect of injecting acid into a rock sample through perforations is studied by using a two-scale continuum model. The model represents the complex coupling between flow, transport, and reaction of acid in a carbonate rock. Two-dimensional simulations of the model show the flow redistribution inside a rock sample when acid is injected through a constriction. It is shown that the amount of acid required to achieve a given permeability (or, wormhole-penetration distance for a given volume of acid) depends on both perforation and sample dimensions. Conditions under which maximum skin decrease is observed are also identified. In addition, the main differences in predictions from perforated and openhole completions are highlighted.
Because a substantial number of current carbonate well completions are cased and perforated, it is important to understand matrix acidizing under such conditions. Particular application from this work includes better matrix-acid treatment designs with optimum acid volumes and rates.
Matrix acidizing of carbonate reservoirs is typically done to enhance the production of hydrocarbons by creating conductive channels through which hydrocarbons can flow into the wellbore and bypass the near-wellbore region. The conductive channels that provide the highway for hydrocarbon flow are called wormholes. Wormhole formation is beneficial because the maximum resistance to flow during production occurs close to the wellbore. Wormholes connect the wellbore to the far-field by establishing direct communication, and thus avoid the radial flow of hydrocarbons around the wellbore. An acid is selected based on the field conditions (mineralogy, temperature, heterogeneity, etc.) and is injected into the formation through the wellbore. Depending on the injection rate, dissolution patterns of different shapes and structures may be created. At low injection rates, the acid residence time in the formation is high and it gets completely spent before penetrating deep into the formation. Dissolution occurs primarily at the formation face and hence, the patterns are called face-dissolution patterns. At high injection rates, the residence time of acid in the formation is low and it does not have enough time to react and therefore, a more uniform dissolution of the formation is achieved. Although very deep penetration is achieved at high rates, the connecting pore throats that are responsible for flow permeability are not necessarily dissolved. At intermediate rates, the reaction and convection rates are such that the formation is dissolved preferentially and leads to the formation of wormhole channels. Because very small amount of rock material is dissolved, the acid volume required to connect the wellbore to far-field is lowest, making wormhole patterns the most desirable dissolution patterns for practical purposes.
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