Mechanisms of Mudcake Removal During Flowback
- Zulkeffeli M. Zain (Petronas Research) | Mukul M Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2001
- Document Type
- Journal Paper
- 214 - 220
- 2001. Society of Petroleum Engineers
- 2.4.5 Gravel pack design & evaluation, 4.1.5 Processing Equipment, 5.3.4 Reduction of Residual Oil Saturation, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 2.7.1 Completion Fluids, 1.11 Drilling Fluids and Materials, 5.3.2 Multiphase Flow, 1.8 Formation Damage, 1.6 Drilling Operations, 2 Well Completion, 5.1 Reservoir Characterisation, 1.6.9 Coring, Fishing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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External filter cakes are used to minimize fluid loss and solids invasion to a formation from drilling and completion fluids. Subsequently, the cake must be removed to increase the flow area and minimize skins, especially for openhole and gravel-packed completions in horizontal and deviated wells. Experimental data is presented to show that the pressure required to initiate flow into the wellbore after building up a filter cake is affected by rock permeability; mud properties, such as particle size and cake yield strength; flowback velocity; and overbalance pressure.
Mudcake liftoff tests, mud particle size, and rock pore-throat size distribution measurements were performed to understand the factors that contribute to cake removal. The mechanisms that control filter-cake removal are discussed.
In this study, it is clearly shown that the flow-initiation pressure during flowback is controlled by solids and filtrate invasion (i.e., internal formation damage rather than by the external mudcake). Flow-initiation pressures show a minimum with increasing rock permeability caused by two competing effects. Larger pore sizes result in smaller flow-initiation pressures, ?Pfi; however, more solids invasion increases ?Pfi. Higher overbalance pressures also increase the internal formation damage and flow-initiation pressure. A simple model to calculate the flow-initiation pressure during flowback is proposed. The model correctly predicts the experimentally observed trends with rock permeability, mud particle size distribution, extent of solids invasion, and yield strength of the mudcake. The model provides a systematic method for designing fluids with low flow-initiation pressures. It is also shown in this study that low flow-initiation pressures do not imply complete cake removal. Cake removal is primarily controlled by the permeability of the cake and its mechanical properties.
Experimental results on the flow-initiation pressure and the return permeability for two commonly used drill-in fluids were presented in an earlier paper.1 It was unclear why the flow-initiation pressure decreases and increases again after reaching a minimum as the core permeability increases. It was also observed that ?Pfi increases linearly with the fluid flowback rate. This work presents additional experiments and analysis to explain the factors responsible for flow-initiation pressure and mudcake liftoff. Measurements of rock pore-throat size and mud particle size distribution were conducted to assist in this explanation.
Mudcake Liftoff Tests.
Static filtration experiments were conducted in a modified high temperature/high pressure (HTHP) API fluid-loss cell that accomodates cores 2.5 in. in diameter and 1 in. long. Details of the experimental apparatus and the procedures for fluid and core sample preparation and data analysis have been reported in an earlier paper.1
The properties of external mudcakes are affected by the circulation of muds under dynamic conditions. However, as shown in this study, we need to be primarily concerned with the invaded solids (i.e., the internal mudcake). Mudcake liftoff tests were performed under different conditions to understand the factors that control the flow-initiation pressure and the mechanisms of filter-cake removal.
Factors Responsible for Flow-Initiation Pressure (Internal vs. External Cake).
To investigate the contribution of external and internal filter cakes (invaded solids) to the flow-initiation pressure, three sets of liftoff tests were conducted. Each set comprised two liftoff tests, one with the external mudcake intact before flowback and the other with the external mudcake removed. Two sets of tests were done with sized CaCO3 mud, with one test each on Texas limestone and Berea sandstone core samples. A third set of tests was done on a Berea sandstone core sample with a bentonite mud system. After the mudcakes were formed, they were removed or mechanically scraped off the surface of the core sample, as shown in Fig. 1. However, a thin layer of mudcake (residue) still remained on the core face because the removal did not involve chemicals or solvents, such as acid, for complete cake removal. The core samples with no mudcake were then reinstalled into the filter cell, followed by flowback with oil (Escaid*) at 5 mL/min. The differential pressure profiles recorded during flowback for these three tests reflect only the behavior of the fluid flow through the damaged core samples with no mudcake sitting on top of the samples. Comparisons were then made with similar liftoff tests in which the mudcakes were not physically removed.
Effect of Two-Phase Flow.
Mudcake liftoff tests were conducted with 3% sodium chloride (NaCl) brine as the flowback fluid instead of oil. The same brine solution was used to saturate the core sample. The test reported here was conducted with sized CaCO3 mud on a Berea sandstone core sample at 100 psi filtration pressure and 170°F. After 16 hours of static filtration, the core sample was assumed to be at a residual oil saturation. The filtrate inside the core sample also contained NaCl brine and a polymer solution. Sized CaCO3 mudcakes built on Berea sandstone were not removed by oil. Cracks and small holes were seen on the cake surface after flowback with oil. The 3% NaCl brine was used as a flowback fluid to see whether two-phase flow effects would change the FIP or improve the cleanup of the mudcake.
Comparison of Bentonite Mudcake With Sized Salt and CaCO3 Mudcakes.
Two mudcake liftoff tests were conducted with bentonite drilling fluids to compare them with the mudcakes of sized CaCO3 mud and sized salt mud. Two drilling fluids, one containing 20 lb/barrel bentonite with 4 lb/barrel lignosulfonate and another containing 20 lb/barrel bentonite with 80 lb/barrel drill solids, were used. Both tests were performed on Berea sandstone core samples at room temperature (70°F). Oil was injected during flowback at 5 mL/min. One of the observations made in an earlier paper1 was that both sized CaCO3 and sized salt mudcakes on Berea sandstone were not removed at all. The effectiveness of bentonite cake removal can, therefore, be compared to sized CaCO3 and sized salt mudcakes.
Particle Size Distribution in the Mud.
The size distributions of solid particles in the sized CaCO3 drill-in mud and a typical flocculated bentonite drilling mud were measured with a laser diffraction method. A comparison of these two mud systems can be made in terms of their particle size distributions (Fig. 2a). Knowledge of particle size can be used to qualitatively infer mudcake porosity and permeability and also the tendency of the pore space to be invaded by the particles.
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