Foam Property Requirements for Proper Hole Cleaning While Drilling Horizontal Wells in Underbalanced Conditions

Abstract

Summary This paper summarizes an extensive experimental program to determine effective foam drilling conditions in horizontal wells. The program included foaming-agent selection (considering optimum concentration and salt/oil contaminants), rheological characterization of foams, development of a flow loop to test the foam-carrying capacity in high-angle wells, definition of the test procedure and matrix, and analysis of the results. After 60 bed tests were performed at a cuttings-transport flow loop, correlations were proposed to predict the cuttings-bed erosion capacity in horizontal wells as functions of the foam quality and the mixture's Reynolds number. Introduction Among the several applications of lightweight fluid drilling, one of the most attractive is the drilling of reservoirs in underbalanced or near-balance conditions. This technique allows the drilling of producing zones with minimum damage. Of course, a good knowledge of hydraulics and cuttings transport mechanics of such complex systems is a must for safe and economical drilling. Nowadays, producing reservoirs in an economical way very often requires the drilling of high-angle, horizontal, and other wells with more complex trajectories. For these well trajectories, several experimental and modeling studies have been done to understand hydraulics and cuttings transport with incompressible fluids.1–4 This paper presents an initial experimental and modeling effort to determine and predict cuttings-transport performance of foamed systems in horizontal and inclined wells. Lab and pilot scale equipment were used as tools for the evaluation of foam stability, rheology, and cuttings-bed erosion capacity. Based on the experiments, comprehensive curve-fitting functions were proposed for explaining the flow- and cuttings-transport behavior observed. This work was conducted as part of the Joint Industry Project "Offshore Drilling with Light-Weight Fluids" (Nakagawa et al.5), which aimed at the drilling of a well with aerated fluid from a floating vessel.6 Foam Stability One major aspect concerning the applicability of a foaming system is the stability of the foam in downhole conditions. An important point to be investigated is the optimization of the system regarding the proper choice of the foamer agent and its optimum concentration. The evaluation of foam stability was performed by an adaptation of a standard method.7 The method is based on the measurement of the liquid phase drained volume as a function of time in ambient conditions. Foams were prepared by stirring 100 mL of the liquid phase for 30 seconds at 5,200 rpm in a controlled-velocity blender. The foam is then immediately transferred to a 1000-mL graduated cylinder, and the drained volume is registered at 1, 2, 3, 5, 7, 10, 14, 20, and 25 minutes. Also, the time (in seconds) when half of the total liquid phase volume (50 mL) is drained was recorded. This value is called the time half-life (t1/2). Effect of Concentration and Type of Foamer. Fig. 1 illustrates the variation of time half-life with concentration for the three foamers studied, called A, B, and C here. The foamers were selected from several commercial products available in the oilfield industry. No chemical characterization or purity analysis was performed or provided by the suppliers. Results show that for the three foamers tested, there is not a direct relationship between increasing foamer concentration and enhancing stability. In all cases, the use of 2% vol/vol foamer concentration resulted in a less stable foam (shorter half-life times) than in smaller concentrations. However, Foamer C showed a higher half-life time than the others. No foam stability testing was carried out under downhole conditions because no equipment was available to operate in such conditions. Effect of Salt Concentration in the Liquid Phase. Another important point to be analyzed is the ability of a foamer to keep foam stable in the presence of salt when brines are the liquid phase. Drilling with brines is a common practice for reservoir hole sections. Fig. 2 illustrates stability results for the three foaming agents with NaCl present in the liquid phase. Results indicate that Foamer A was stable, while the others showed half-life times dependent on the salt concentration. Foamer C half-life times were comparable to Foamer A with less than 5% wt/vol NaCl but increased for salt concentrations greater than 5% wt/vol. Foamer B seemed inadequate to generate stable foam in the presence of salt, which is indicated by half-life times that dropped dramatically with an increase in the salt concentration. Effect of Oil Concentration. The applicability of the three foaming agents for drilling in underbalanced conditions in oil reservoirs was evaluated. Fig. 3 compares the behavior of the half-life time for the three foaming agents in the presence of crude oil. Results indicate that both Foamers B and C were stable in the presence of 30% oil by volume, while Foamer A began losing its stability in concentrations of greater than 20% oil by volume. Comments. Choosing a foamer for drilling operations requires the proper definition of the application for which it is being designed. Of the three foamers tested, Foamer C proved to be applicable for a wider range of situations. Foamer B showed some restriction to use in the presence of salt, while Foamer A showed poor results in the presence of oil. For the flow-loop tests described in the following sections, Foamer B was used, owing to good results and operational convenience. Foam Carrying Capacity in Horizontal and High-Angle Wells Cuttings transport is a major issue in drilling high-angle and horizontal wells. In these situations, there is a tendency to deposit a solids bed in the lower part of the annulus. The excessive amount of solids may create problems, such as abnormal torque and drag, the necessity of redrilling, and, in many cases, drillstring sticking. The objective of this section is to describe the experimental work performed at a pilot-scale cuttings-transport flow loop on bed erosion with foamed fluids in horizontal and inclined wells. Cuttings-bed erosion capacity was measured for several combinations of gas and liquid flow rates at three different inclinations.

Cuttings Transport with Foam in Highly Inclined Wells at Simulated Downhole Conditions / Transport urobku wiertniczego przy użyciu piany w silnie nachylonych otworach w symulowanych warunkach w otworze

“…Extensive experimental work was conducted that considered foaming-agent selection and optimum concentration, salt/oil contaminants, rheological characterization of foams, develop-ment of a flow loop to test the foam-carrying capacity in high-angle wells, definition of the test procedure and matrix, and analysis of the results (Martins & Lourenco, 2001). After 60 bed tests were performed in a cuttings-transport flow loop, correlations were proposed to predict the cuttings-bed erosion capability in horizontal wells as functions of the foam quality and the mixture's Reynolds number.…”

Using foam in horizontal well drilling: A cuttings transport modeling approach

“…They observed the worst hole cleaning performance at angles 40-608. Martins et al (2001) experimentally studied effective hole cleaning with foam. They developed empirical equations to predict the bed erosion capacity in horizontal wells as a function of foam quality and Reynolds number.…”

Rheological behavior of oil-based drilling foams

“…Although field tests (Seulveda et al, 2008;Kakadjian et al, 2012) demonstrate the benefits of oil based foams, rheology and hydraulics of these fluids are not well understood. Accurate rheology predictions improve hole cleaning and ensure the safety of foam drilling operation (Martins et al, 2001b). Hence, this study is focused on rheological characterization of oil-based foams.…”