Pilot-Scale Experimental Study and Mathematical Modeling of Buoyant Settling of Immiscible Heavy Fluid in Mud To Stop Annular-Gas Migration Above Leaking Cement

Abstract

Summary Annular casing pressure (ACP) is defined as the accumulated pressure on the casing head. If pressure returns after bleed-down, then the casing annulus is said to be showing sustained casing pressure (SCP). SCP is caused by late gas migration in the annular-fluid column above the top of leaking cement and may result in atmospheric emissions or underground blowouts. Removal of SCP is required in places where SCP is regulated, particularly before the well-plugging and abandonment operations. Annular-intervention methods for SCP removal, which are less expensive than the conventional downhole-intervention methods, typically involve injecting heavy fluid into the affected annulus that would displace the annular fluid (AF), balance the pressure at the top of cement, and stop the gas leakage. Previous studies stated that the use of immiscible combinations of two fluids is more effective for the purpose; however, inattentive applications may result in excessive use of heavy fluid. In this study, a 20-ft carbon-steel pilot-well annulus was manufactured and used for displacement experiments with various water-based drilling muds and heavy fluids with different properties. Pressure-change data were collected from four different levels of the annulus, and volumes of fluids going in and out of the annulus were measured. Experiments indicated the formation of a mixture zone that would build bottoms up and expand during ongoing displacement. The proposed pressure-buildup model suggests an exponential distribution of density of this zone, and shows its high dependency on fluids’ properties and injection rate. The mathematical models were also converted into dimensionless process measures and proposed for use in real-well applications. The study demonstrates the viability and recommends the correct application of the method.

Design and field-scale demonstration of the buoyant-kill process for restoring integrity of wells with sustained casing pressure

“…Also, the emulsified Flip-flop settling experiment with KF (black) and AF from Table 1. (Demirci andWojtanowicz, 2016b, and.…”

“…The KF injection was studied by considering injection geometry and the rate of stable settling of KF in the stagnant column of AF. The study involved bench-top testing with the slot model of annulus, described above (Demirci and Wojtanowicz, 2015;Demirci and Wojtanowicz, 2016a) and the pilot-scale testing with the annular 30ft tall model, also mentioned above (Demirci and Wojtanowicz, 2016b). The slot-model tests utilized KFs and AFs with different viscosities and densities.…”

“…[8.329-in. inner diameter] carbon-steel pipe (Demirci and Wojtanowicz, 2016b). A total of 37 experiments were conducted with water-based AFs and brominated organic KFs with densities from 9 to 13 ppg and from 17 to 23 ppg, respectively.…”

“…A dimensionless parameter, R u , is the ultimate displacement and represents the ratio of required kill fluid volume to annular volume for maximum annular pressure increase. The parameter value was empirically correlated using the experimental data (Demirci and Wojtanowicz, 2016b) as, R u 10 −0.376 ρ 0.377 af ρ 0.035 kf τ 0.019 0 q 0.1719…”

“…Heavy and highly hydrophobic brominated organics were theoretically selected physically manufactured and benchtop tested . Also, prior to the field-size testing, efficiency of buoyant displacement was investigated in the pilot-scale 30-foot high physical model (Demirci and Wojtanowicz, 2016b). The study provided the bottom pressure and volumetric data for a mathematical up-scaling model used to design the demonstration experiment in field-size well.…”