Investigation of stage ordering impact on fracture growth in multi-stage hydraulic fracturing of horizontal wells in stacked plays

Abstract

Resource extraction and utilization from unconventional reservoirs have become the main focus of the oil and gas industry in the past few decades. Advances in technological capabilities made the unconventional reservoirs congested with horizontal wells with multi-stage hydraulic fracture completions as the main method for recovery. This research aims to investigate the optimum hydraulic fracture treatment schedule on a multi-well pad in stacked pays in the Niobrara and Codell formations of the DJ basin. A 3D numerical hydraulic fracture model was constructed to assess the impact of fracture sequencing and scheduling on resource accessibility and fracture deliverability. The base model consisted of nine wells with a total of 501 hydraulic fracture stages completed in the Niobrara chalk intervals and Codell sandstone. Diagnostic fracture injection tests (DFITs) and well logs were used to calibrate the model. In this study, twenty-five scenarios were created from the base model as a sensitivity analysis to investigate the effect of fracture sequencing and treatment stage scheduling and location in original reservoir pressure condition and in pressure depleted reservoir condition. The fracture sequencing examined different methodologies followed in the industry of consecutive fracturing, alternating two-step fracturing and zipper fracturing. The sensitivity of treatment stage scheduling and location investigated the impact of different treatment orders to complete the nine wells in the multi-well pad on fracture growth and propagation. In the original reservoir pressure condition, where all of the wells in the multi-well pad are completed before the start of production, the optimum treatment stage ranking methodology aimed to ensure that the resulting hydraulic fractures dimensions from the treatments reached their designed potential. The stage ranking method prioritized wells located near stress barriers or in high stress zones, starting with the middle well or wells if a group of wells are located in these areas. This method followed zipper fracturing sequence between the stacked wells. Simultaneous completion for all wells, i.e. completing all nine wells together in a zipper fracturing fashion, had the highest average values of fracture dimensions. Combining the ranking prioritization approach with the zipper fracturing sequence produced the optimum results for fracture dimensions and asymmetry. In the depleted reservoir condition, two wells out of the nine were put on production before the commencement of the other wells’ hydraulic fracturing treatments, dividing the group into two parent wells and seven child wells. The objective of the optimized stage order schedule was to reduce hydraulic fractures propagation into the pressure depleted zones and generate more contact with unstimulated rock. The stage ranking prioritization aimed to alter the induced stress field created by the stress shadowing effect in order to increase the total stress in the depleted areas and divert the proceeding fracture treatments away from the pressure depleted zones. This was accomplished in the simulation by placing the nearest well to the parent wells and located in-between the parent wells and the child wells at the top of the treatment schedule. Consecutive fracturing for the first child well in this methodology was followed in order to create a stress field around the depleted zones that acts as a stress barrier to the subsequent fracture treatments. The following wells in the optimized stage scheduling system followed zipper fracturing and were ranked based on their proximity to depleted zones. The simulation results also indicated that reducing the treatment volumes of the nearest well to the parent wells or to the depleted zones would minimize the propagation of the hydraulic fractures in these areas and still generate the required stress field around them to divert fracture propagation from other treatments. This study focuses on the effect of hydraulic fracture stage ordering and sequencing by holding all other aspects of the hydraulic fracture treatments constant, in order to isolate the treatment scheduling as the only changing parameter. The results showed that optimizing treatment schedules in original pressure state conditions leads to an optimum outcome from the stimulation. In pressure depleted conditions, optimizing treatment scheduling alone does not provide a complete solution to the parent/child well problem. However, this research showed that treatment order and scheduling is an essential part that should be integrated with other solutions such as protection fracs and wellbore re-pressurization.