Horizontal Hydraulic Fractures: Oddball Occurrences or Practical Engineering Concern?

Abstract

Abstract Conventional wisdom1 regarding horizontal hydraulic fractures is that they are common in shallow environments but that they generally do not occur below a "critical" depth of about 2000 ft. However, direct measurement of hydraulic fracture orientation utilizing tiltmeter fracture mapping on well over 1000 fracture treatments has shown that hydraulic fracture growth behavior is much more complex than implied by this simple "rule of thumb". Horizontal hydraulic fractures are far more common than generally believed. This paper documents the widespread occurrence of horizontal fractures in two classes of environments: (1) reservoirs with high horizontal stresses, including an example at 7,500 ft.; and (2) reservoirs undergoing Enhanced Oil Recovery (EOR) where the distribution of the overburden load (vertical stress) has been altered such that horizontal fractures are created even when fracture pressure gradients are well below the overburden gradient as estimated from an integrated density log. Fracture mapping data is presented from three different fields. The Lost Hills Field in California exhibits a curious stress state where the shallowest zones at the top of the thick diatomite reservoir (1000-2000 ft.) yield near vertical hydraulic fractures, but stimulating the deeper zones often creates horizontal hydraulic fractures due to a stress state "reversal" with depth. Fracture orientation data is also presented from the 7,500 ft. deep North Shafter Field where one treatment resulted in a (near) horizontal hydraulic fracture(s) that resulted in a very early premature treatment screenout, in contrast to another nearby fracture treatment where a dominant (near) vertical fracture was created and there was no difficulty placing all 400,000 lb. of proppant. These two fields illustrate "unconventional" original insitu stress states that result in significant horizontal fracture growth at depth. The third example presented, in the massive Belridge oil field, illustrates a much different phenomenon where the original stress state generally resulted in near vertical hydraulic fractures throughout the entire interval. However, secondary recovery has sufficiently altered the stress state such that horizontal fracture growth is becoming very significant. Direct fracture intersection of nearby wells has confirmed the creation of horizontal hydraulic fractures in many wells in the Belridge Field, even when the observed fracture gradients are well below the integrated density log estimates of the overburden stress. We believe that this is due to a "room and pillar" vertical stress state that is created by the highly variable reservoir pressure profile induced by secondary recovery (waterflooding), with a lower local vertical stress around producer wells and higher vertical stress around injector wells. Figure 1 illustrates the tremendous impact that horizontal hydraulic fracture growth can have on hydrocarbon recovery for both primary and secondary production. Horizontal fractures can severely reduce fracture treatment coverage in thick gross pay intervals, particularly when vertical permeability is impaired. In secondary recovery, horizontal fractures can greatly reduce water/steam flood sweep efficiency and leave large parts of the reservoir unswept. Horizontal fractures can also be far trickier to place proppant in and, therefore, lead to many premature treatment screenouts. Although these issues are rarely considered2, fracture and completion design should be significantly altered if horizontal fracturing is occurring.