Multiple Precision Hydraulic Fractures of Low-Permeability Horizontal Openhole Sandstone Wells

Abstract

Abstract Many horizontal wells with openhole completions have been drilled in low-permeability sandstone formations. The Cardium formation in west-central Alberta, Canada is one such case. Many of these wells perform below predicted rates with uneconomical gas production. Until recently, no method existed to effectively fracture stimulate this type of wellbore to optimize production. Possible reasons for low production rates in these wells include vertical heterogeneity of the sandstone, low horizontal permeability, and much lower vertical permeability. The heterogeneities in the sandstone can act as barriers to vertical flow. Near wellbore damage is another possible reason for low production. One way to neutralize the effect of these barriers or damage is to hydraulically fracture the formation, effectively connecting the full porous height of the formation to the wellbore through a propped vertical fracture. Nevertheless, if only one hydraulic fracture is placed, the horizontal wellbore may not produce more than a vertical well. A new hydrajet-fracturing technique, however, now exists whereby many propped hydraulic fractures can be placed individually at strategically selected locations along an openhole wellbore. The resulting horizontal wellbore with multiple propped fractures can potentially produce gas at rates higher than those of a vertical well. This paper briefly describes the hydrajet-fracturing process and documents two case histories from the Cardium formation in which this system was applied. Both wells were hydraulically proppant fractured at multiple locations precisely placed along their openhole horizontal wellbores. Introduction Horizontal wells in low-permeability rock typically produce at lower than expected rates for various reasons. There has been a common misconception that horizontal drilling will eliminate the need for the stimulation treatments required for vertical wells within the same area. It is also mistakenly believed that near-wellbore damage in horizontal wells is of less concern than in vertical wells, and that most reservoirs are homogeneous with few disparites in vertical permeability. In some cases horizontal wells are drilled in known heterogeneous formations with the intent of intercepting high permeability lenses or natural fractures. In a significant percentage of these cases, the resulting gas production is disappointing, especially if the targets are missed, even narrowly, by the wellbore. For most of North America, the average permeability of the pay in new wells is decreasing, as the best pay has been drilled out and more marginal pay is sought. Certain points should be kept in mind with regard to low-permeability horizontal wells:Low-permeability rock typically contains vertical heterogeneities that limit or prevent effective communication of the porosity with a horizontal wellbore. These heterogeneities can be due to porosity and permeability variations or to small laminations of nonpermeable minerals. The ratio of vertical permeability to horizontal permeability is usually low in low-permeability rock, thereby impeding the vertical movement of hydrocarbons. It only requires one thin nonpermeable lamination to totally block vertical flow to the horizontal wellbore, and many laminations are too thin to be detected by conventional logging tools.Many horizontal drilling projects proceed without a review of producing rock cores or high-resolution logs. Core samples from low-permeability sandstone reservoirs, especially gas reservoirs, often indicate that full-core vertical permeability can dramatically differ from test plugs drilled vertically (Fig. 1)In many low-permeability reservoirs with openhole laterals, skin damage values are often quite high compared to those in moderate- or high- permeability areas.Many damage-removal treatments have little effect in low-permeability sandstone formations, especially with long openhole laterals, where they may have been used in attempts to offset the need for fracture stimulation treatments.