Stimulation Fracture Height Control Above Water or Depleted Zones

Abstract

Abstract Many potentially productive zones located a few meters (feet) above a water or pressure-depleted interval are not fracture-stimulated, or are eliminated from further investigation rather than risk inter-zone communication. In either case, production fails to be optimized and often recoverable reserves are lost. Several techniques for controlling formation fracture height growth have been successfully field-tested. The fracturing fluid used was either water- or oil-based with a low viscosity of 20 to 50 cp. A general program, utilizing a timed sequence of rate and proppant concentration control, is preceded by application of fracture height-controlling material in the pad. Approximately 5 tonne (11,000 lb) of proppant were consistently placed with total fracture height growth traced to less than 15 m (50 ft). Post-stimulation well testing indicated fracture lengths to 90 m (300 ft) in reservoirs having permeabilities as low as 0.1 md. Fracture modeling using a numerical simulator prior to actual field stimulation has proven to be instrumental in designing an optimal fracture program. This unified procedure prevents or minimizes fracture height growth, from initial breakdown of the formation to the flow period after pumping. Well stimulation is performed through tubing to reduce fluid volumes. Pumping rates ranged from 0.5 to 1.5 m 3/min (3.1 to 9.4 bbl/min). To date, ten treatments have been successfully placed and there is no evidence to suggest that fracture heights were greater than those predicted by computer simulation. Introduction For optimized well performance by a fracture stimulation program, adequate fracture half-length and fracture conductivity are the two most important parameters. Fracture half-length is more of a concern in lower permeability zones, and since fracture height varies inversely with fracture length, the lower the fracture height, the greater the fracture length (for the same net pressure). An operating necessity is therefore to control the height of a fracture in the formation zone of interest. Fracture height control has been studied for more than thirty years.1 Barrier in situ stresses of 1.4 to 4.8 MPa (200 to 700 psi) can slow down or halt fracture height growth. 2 Hanson et al 3,4 investigated control of the fracture height when alternately increasing and decreasing injection rate and proppant concentrations and, by introducing deformable solids having a density near that of the fracturing fluid, to plug paths of least resistance. Arp et al 5 reported improved production by designing upper and lower fracture height control, using lighter proppants (upper control) and a mixture of silica flour, 70/140 mesh, 20/40 sand, and 10/20 sand (lower control), at 1.5 tonne (3,300 lb) each at 0.8 m3/min (5 bbl/min). After a forty-five minute shut-in, the main treatment of 27 tonne (59,400 lb) of 20/40 proppant was pumped at 1.1 m3/min (7 bbl/min). Other diverter schedules varied pumping to one-half the rate of the main fracture treatment. Arp et al 5 criticized the use of low rate-low concentration fracs claiming such programs yielded less fracture conductivity and thus, less well productivity. The debate follows from the assertion that dimensionless fracture conductivity (FCD) is sensitive to both fracture length and fracture width: Equation 1 where, kf is the effective fracture permeability, wf is the propped frac width, k is the effective formation permeability, and Xf is the fracture half-length. Low proppant concentrations or extra fracture height tend to produce smaller fracture widths and hence less fracture conductivity. Cipolla et al6 demonstrated that wells stimulated with sand and having extra frac height growth outside the pay zone produced more through improved fracture conductivity. Using values of twice the formation thickness (hD=2) gave an increase in gas production of about 5%, while a frac height of hD=5 resulted in a production increase of 10%. For manufactured proppants, the incremental production due to fracture height growth was less than 1%.