Novel Frac-and-Pack Technique for Selective Fracture Propagation

Abstract

Abstract Frac-pack is a pervasively used completion technique in wells targeting high permeability, poorly consolidated and depleted sandstone formations located in Bachaquero, TÍa Juana and Lagunillas fields in West Venezuela. This technique combines stimulation and sand production control in a single treatment by placing a short and wide fracture which bypasses the near-wellbore damage, while gravel-packing the zone of interest. This paper describes a novel and economical frac-and-pack technique which consists of pumping a sand plug with the downhole tool set for circulation to isolate a bottom set of perforations, followed by conventional frac-and-pack. When this procedure is followed, the fracture is forced to propagate along the upper intervals. This novel technique is particularly useful for wells with water-producing zones near the bottom of the target zone, because it induces selective growth of the fracture along the upper intervals and mitigates the risk of growing the fracture into the water-producing zone. A case study of a frac-and-pack performed in a Lagunillas field well with a water contact 40 ft below the target zone is reviewed. The intervention rendered an increase in well production rate from 27 to 173 net barrels per day with a reduction in water cut from 25% to 9%. In contrast, two wells in the same field and with very similar characteristics which were frac-and-packed conventionally rendered 100% and 63% water cuts, respectively. Another application of this technique refers to frac-and-pack of wells with long perforated intervals where early wellbore screen-out may occur due to proppant bridging of the annular volume between the screen and the casing. Conventional frac-and-pack of twenty wells in these fields with perforated intervals exceeding 90 ft rendered a 40% early wellbore screen-out rate. The early wellbore screen-out rate was reduced to 12% in a sample of twenty eight wells with the new technique. The average production rate increased from 2 to 135 BOPD, whereas the average estimated after-treatment production was 130 BOPD, for which this technique was considered successful. A shortcoming of the technique for this application is the fact that the bottom of the perforated interval is not fractured. High-end frac pack techniques that overcome this issue such as use of shunt tubes were found to render higher normalized oil production rates. Introduction Frac-and-pack was first implemented in Venezuela in the early 1960s 1. Small scale stimulations were performed using crude oil as fracturing fluid with gravel sized to control production of formation sand. Ball sealers were used in long perforated intervals as diversion mechanism to achieve better zone coverage. After the treatment was performed, a screen was washed down through the gravel remaining in the casing and additional sand was placed around the screen. Frac-and-pack became a premier completion technique in the late 1980s 2 when the use of the tip-screenout (TSO) technique earned worldwide acceptance as an effective stimulation method for high permeability formations. Modern frac-and-packs involving the use of the TSO technique, high proppant concentrations and gelled fracturing fluids were first introduced in Venezuela by Schlumberger in 1996. Since then, this technique has become the preferred option to stimulate unconsolidated sandstones with low reservoir pressures. Currently, 40% of stimulations performed in unconsolidated sandstones for Petroleos de Venezuela S.A. (PDVSA) in West Venezuela are frac-and-packs. The importance of the frac-and-pack technique relies on the fact that less rig time is required with respect to a job for which stimulation and gravel packing are performed in separate stages. Therefore, the client cost for the workover operation is significantly reduced. Frac-and-pack also eliminates the risk of damaging the near-wellbore properties of the fracture with well cleanout operations required after fracturing and before gravel packing, when both are performed separately.