Polymer Replacement for Organophilic Clay Benefits Mineral-Oil-Based Drilling Fluids
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Organophilic clays mixed in oil-based drilling fluids (OBDFs) do not exhibit the same viscosity or suspension characteristics as they do in water-based drilling fluids because electrical interaction between particles is minimal, creating difficulty in building viscosity and gel strength under high-pressure and high-temperature (HP/HT) conditions. To address that problem, a new mineral-oil-based drilling fluid (MOBDF) was created by replacing the conventional organophilic clay with a novel polymer.
Organophilic clays are widely used as primary viscosifiers for OBDFs and synthetic-based drilling fluids. These chemicals are prone to stratification in certain conditions, slow chemical-reaction times, high pressure spikes, and high equivalent circulating densities (ECDs). These mud systems also have temperature limitations. At high temperatures, thermal thinning and thermal degradation of their organophilic colloids permanently destroy their viscosification capabilities. At low temperatures, excessive and rapidly increasing viscosity is experienced.
A need, therefore, exists for a fluid that produces higher gel strengths initially and maintains a relatively flat response to temperature throughout the complete temperature cycle of a drilling operation. Organophilic clays mixed in OBDFs do not exhibit the same viscosity or suspension characteristics as they do in water-based drilling fluids because electrical interaction between particles is minimal, creating difficulty in building viscosity and gel strength under HP/HT conditions.
By switching from a standard OBDF containing organophilic-clay additives to a clay-free OBDF, the drilling operation will benefit from a drilling fluid capable of performing under the desired conditions planned, an overall reduction in logistics, a reduction in volume of fluid and material needed, higher rates of penetration because of lower ECDs, and the formation of a thin nondamaging filter cake that would eliminate the need for costly and time-consuming cleanup treatments.
This paper studies the feasibility of developing an organophilic-clay-free drilling fluid capable of performing similarly to an already-tested drilling fluid containing organophilic-clay additives.
Two MOBDF formulas were used for the experiments. The first, MOBDF A, was prepared with a formula containing organophilic-clay additives. The second, a clay-free drilling fluid, MOBDF B, was prepared with a formula containing the replacement polymer viscosifier and filtration-control agent.
Calcium carbonate was chosen as the weighting material for the two MOBDFs used for the analysis.
High-permeability Indiana limestone cores with a thickness of 0.25 in. and a diameter of 2.5 in. were used to perform the filter-cake-formation and filtration analysis.
Berea sandstone cores with a thickness of 6 in. and a diameter of 2.5 in. were used to perform the rock-wettability and emulsion-breakdown analysis.
Both MOBDFs were heated to 150°F for 16 hours under hot rolling, and no phase separation was observed.
Results and Discussion
Rheological Properties of MOBDF A and MOBDF B. Polymer A was used as a viscosifier and filtration control agent in MOBDF B. The performance and stability of MOBDF B was examined and compared with the performance of MOBDF A. The density of both drilling fluids was kept constant for all the experiments and measured at room temperature. The rheology was resolved with the Bingham plastic and power-law models. Properties were measured at 120°F.
Filtration-Control Comparison of Organophilic Clay and Polymer. Filtration tests were conducted using an HP/HT filter press under static conditions. Both drilling fluids were put in the cell, and the temperature and pressure were adjusted to 140°F and 300 psi, 190°F and 300 psi, and 250°F and 500 psi. High-permeability Indiana limestone cores with a thickness of 1 in. and a diameter of 2.5 in. were used for this experiment. First, the filter cake was formed at the temperature and pressure. The filtrate was collected over a 30-minute interval, and the results show that the fluid loss was more prominent for MOBDF A.
Filter-Cake Porosity and Permeability. The Indiana limestone cores were scanned before each experiment in dry and wet conditions to determine their initial porosity.
- Dry Conditions—Each core was dried in the oven for 16 hours at 150°F.
- Wet Conditions—Each core was saturated with a 5 wt% potassium-chloride (KCl) brine for 48 hours before the day of the experiment.
Effects on Permeability of Formation. Coreflood tests were performed to determine the removal efficiency of the filter cakes generated by MOBDF A and MOBDF B at a temperature of 250°F. The backpressure and overburden pressure were set at 1,100 and 1,600 psi, respectively. Indiana limestones cores were used for the test. The initial permeability of each core used in these experiments was measured using Darcy’s law.
The time required to flow 1 pore volume (PV) of 5 wt% KCl brine at a constant pressure was recorded. The same procedure was performed after the removal of the filter cake to calculate the final permeability.
The core PV was calculated from the density of the brine and the weight difference in both dry and saturated cases of the Indiana limestone cores.
Filter-Cake-Removal Efficiency. The efficiency of removing the filter cake was acquired by measuring the weight of the Indiana limestone core after being saturated in the 5 wt% KCl brine for 48 hours. After forming the filter cake at the desired temperature and pressure with the MOBDFs, the weight of the core was measured a second time. A third and final weight measurement was taken after soaking the core in the removal solution for 6 hours.
- A clay-free MOBDF formula (MOBDF B) was developed to replace organophilic clay additives with a cross-linked polymer as a viscosifier and filtration-control agent.
- On the basis of the results obtained from the rheology tests performed on the clay-free MOBDF, the authors conclude that the viscosity and suspension characteristics of this drilling fluid were better compared with a drilling fluid containing organophilic-clay additives. The apparent viscosity of MOBDF B was lower than the apparent viscosity of MOBDF A when subjected to different temperatures. MOBDF B also exhibited good ability to suspend cuttings at different temperatures and pressures.
- The clay-free MOBDF exhibited good filtration properties, a thin filter cake, a small filtrate volume, a small spurt loss, and a filter-cake permeability of less than 0.04 md.
- The removal efficiency of the filter cake generated by the clay-free MOBDF was very similar to the removal efficiency of the drilling fluid containing organophilic-clay additives.
- After filter-cake removal, the retained permeability measured for the clay-free MOBDF was higher than the retained permeability attained after removing the filter cake formed by the drilling fluid containing organophilic-clay additives.
For a limited time, the complete paper SPE 191776 is free to SPE members.
Polymer Replacement for Organophilic Clay Benefits Mineral-Oil-Based Drilling Fluids
01 November 2018
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