Valhall Field: Horizontal Well Stimulations "Acid vs. Proppant" and Best Practices for Fracture Optimization

Abstract

Proppant fractures along the horizontal laterals in the Valhall Field have become a standard completion method for the last eight years with over 150 proppant fractures completed to date. The development of the flank regions of the Valhall Field began, with the first of 14 wells completions, in March 2003. Lateral lengths of up 2,000 meters will be drilled from two new platforms placed on the North and South edges of the field. The chalk formation in the flank regions is expected to be more competent then the crestal part of the field, so the question was raised as to whether fracturing should be done with acid or proppant. From a proppant fracturing perspective, each flank well will require between 10 to 14 prop fractures along its lateral requiring 2.5–3.7 Million pounds of proppant per well. Three different methods have been used to determine whether the wells should be acid or proppant fractured. These consist of reviewing the historical well performance, analytical and numerical modeling. All three methods clearly showed proppant fracturing was the preferred stimulation for the Valhall Field regardless of it's location. Acid fracturing becomes the stimulation of choice only if the well does not come in contact with enough OOIP (e.g. 5 MMSTB) to justify the proppant fractures. Proppant fracturing is expensive, so in conjunction with identifying the best stimulation method for the flank region, optimization with respect to fracture spacing along the horizontal lateral, fracture length and width have been numerically modeled for both the crestal and flank wells. This is an evolving process that should be considered an industry "Best Practice" as it enables real time optimization of ‘prop’ fracturing along a horizontal lateral during the drilling and completion phase. Since the start of this "Best Practice" in 2002, the wells stimulated in the crestal part of the Valhall Field have had the highest productivity in the field's history. Introduction The Valhall Field is an Upper Cretaceous, asymmetric chalk anticline that forms an overpressured, under-saturated, oil reservoir located in the Norwegian sector of the North Sea (Figure-1). It is characterized by high porosity (25–48%) and high oil saturation (92–97%). In common with other chalk reservoirs in the region, the drive mechanism is a combination of fluid expansion and formation compaction. The production is mainly from the high porosity (35–48%) Tor formation, which exhibits typical thickness of 15–40 m and a permeability of 1–10 mD. The source rock is the organically rich Kimmeridigian Clay underlying the chalk and has resulted in a reservoir, which is over pressured for depth with an undepleted gradient of 0.78 psi/ft TD at 2,500 mTVD1. The field will recover over 1 billion STB with current production of 80,000 BOPD from 42 wells on the main Drilling Platform and the Wellhead Platform (WP), which are centrally located in the crestal part of the field. The field was originally sanctioned and placed on production back in 1982 with reserve estimates of only 250 MMSTB. Reserves have increased by 400% since then throughbetter reservoir description,improving the completions and drilling strategy from vertical indirect proppant fractures to long horizontals with multiple proppant fractures, andthe current development of the flank areas of the field and the initiation of a waterflood in the crestal section.

Improving North Sea Fracturing and Stimulation Economics Through Increasing Innovation and Efficiency in Operations and Applications: Lessons Learned

“…This technique remains the predominant completion method applied in the Valhall Tor reservoir. The success of this method, from a production sense, has previously been described in detail (Olsen et al, 2003).…”

Seismic-to-Simulation for Unconventional Reservoir Development

“…Discrete gridding the hydraulic fracture in a numerical reservoir simulation model is the most robust and flexible approach, but can be a time consuming and cumbersome exercise if the process is not automated. Automated gridding of multiple planar hydraulic fractures in horizontal wells has been possible for many years (Shaoul et al 2005), but the application of these integrated approaches has been limited (Olson et al 2003, Lolon et al 2007). However, the integration of complex hydraulic fracture models with numerical reservoir simulation has been a manual and time-consuming process, with limited application (Cipolla et al 2010).…”

“…Generating these complex reservoir simulation grids and accurately populating the hydraulic fracture properties is an overwhelming process if done manually. Although industry solutions for automatic grid generation for multiple planar hydraulic fractures in a horizontal well are available (Olsen et al 2003(Olsen et al , 2004Shaoul et al, 2005), solutions for complex hydraulic fractures have been absent until recently. Fig.…”

Valhall Field: Microfracture Acquisition Using Wireline-Formation-Tester Tools To Validate Fracture Confinement

“…1 In April 2003 the development of the North and South Flank areas of the field started and to date 12 wells have been completed, of the those wells, five have had reservoir pressures close to original (6450 psi). Front end loading work prior to the Flank Development identified the need to understand whether proppant fractures would stay confined when the reservoir pressure was close to original.…”