Predicting and Monitoring Fluid Invasion in Exploratory Drilling
- Alex T.A. Waldmann (Petrobras) | Andre L. Martins (Petrobras S.A.) | Atila F.L. Aragao (Petrobras S.A.) | Rosana F.T. Lomba (Petrobras S.A.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2005
- Document Type
- Journal Paper
- 268 - 275
- 2005. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 5.3.2 Multiphase Flow, 5.8.6 Naturally Fractured Reservoir, 2 Well Completion, 5.6.4 Drillstem/Well Testing, 3 Production and Well Operations, 5.2.2 Fluid Modeling, Equations of State, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 2.7.1 Completion Fluids, 1.12.2 Logging While Drilling, 5.1.1 Exploration, Development, Structural Geology, 1.8 Formation Damage, 5.2 Reservoir Fluid Dynamics, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.7.1 Underbalanced Drilling
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This study proposes, through the coupling of a linear filtration formulation(laboratory configuration) and a radial single-phase formulation (wellborevicinity), to predict fluid-invasion depth-of-drilling fluid filtrate in thereservoir rock. Modeling is validated with linear and radial laboratory tests,as well as with resistivity logs run in offshore wells from Campos basin,offshore Brazil.
The proposed methodology is required for optimum drilling-fluid design to beused in the drilling of reservoir sections in both exploratory anddevelopmental wells in Campos basin.
Minimizing fluid invasion is a major issue when drilling reservoir rocks.Large invasion may create several problems in sampling reservoir fluids inexploratory wells. Unreliable sampling may lead to inaccurate reservoirevaluation and, in critical cases, to wrong decisions concerning reservoirexploitability.
In addition, drilling-fluid invasion may also provoke irreversible reservoirdamage, thereby reducing its initial and/or long-term productivity. Suchproblems can be critical in heavy-oil reservoirs in which oil and filtrateinteraction can generate stable emulsions. Invasion in light-oil reservoirs isless critical because of good mobility properties.1Another critical scenario is low-permeability gas reservoirs in whichimbibition effects may result in deep invasion.
To avoid these problems, the drilling-fluids industry spends a lot of effortproviding noninvasive systems (Reid and Santos2 and Luoet al.3, among several others). A common practice in theindustry is to add bridging agents such as calcium carbonates into thedrilling-fluid composition. Such products would form a low-permeability layerat the well walls, which would control invasion. Several authors presentfluid-composition-optimization studies for specific situations (Krilov etal.4, among others).
Field Evidence of Drilling-Fluid Invasion
The evaluation and diagnosis of the cause of formation damage is a complexand important topic because it may define effective or noneffective stimulationmethods. It is frequently difficult to define whether drilling-fluid invasionwas or was not a cause for a bad productivity index result.
Direct evaluation of filtrate invasion is also an important and complexissue. Logging-while-drilling (LWD) and wireline logging are the most effectivetools available. LWD-resistivity responses establish the initial situation inthe well, while logs provide semiquantitative invasion information regardinginvasion at different distances from the well at the end of the drilling phase.Formation test results indicate invasion when the sampled fluid containsfiltrate; however, it may be imprecise to use such data to predict radialinvasion caused by permeability-anisotropy effects. In this case, filtratecould be migrating vertically from the reservoir directly to the samplingchamber.
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