In-situ Stresses And Hydraulic Fracturing In the Deep Basin
Authors: Kry P.R. , Gronseth J.M. 
Published: March 2010

Hydraulic fracturing is a means of developing low-permeability gas sands. The most significant parameter controlling created fracture geometry is in-situ stress slate. Measurements in the Deep Basin have demonstrated differences of 6 MPa in the minimum horizontal in-situ stress over 15 – 30 m with distributions which are both favourable and unfavourable to fracture containment within a producing zone. Conversely, stress variations less than 2 MPa have been observed over a 90 m depth range which contained both a producing interval and thick bounding shale beds. The degree of hydraulic fracture containment has been observed to be related to the vertical profile of the minimum horizontal Stress.

In this region the maximum horizontal principal stress is estimated to be nearly equal to the vertical stress at depths below 2 km. It is about 1.4 times the minimum principal stress. This large contrast suggests that fracture azimuth should be controlled completely by the in-situ stress state in the Deep Basin.

Introduction

Low-permeability reservoirs such as tight gas sands in the Deep Basin of Western Canada are generally uneconomic because of low production rates, not insufficient hydrocarbon reserves. Current technology for increasing these rates relies on hydraulic fracturing. Effective hydraulic fracturing creates a highly conductive channel contained primarily within the productive zone and extending as far as possible away from the wellbore. When a fracture is not contained primarily within the productive zone, an excessive amount of fracturing fluid is used. This has some potentially serious consequences. Increased formation damage may result from excessive fluid leak-off(1), improper proppant placement, such as settling below the zone is possible, and excessive fracture conductivity damage can occur from having to recover extra fracturing fluid(2,3).

The in-situ stress state is the most significant parameter controlling fracture containment. An induced fracture can be contained within a zone having a lower stress than that in the bounding layers. Laboratory experiments have shown that under appropriate conditions stress differences of 2–3 MPa are sufficient to contain a fracture(4,5). Field observations have shown stress variations of 6–8 MPa over short" vertical distances(6,7). Fracture containment was observed under appropriate conditions.

This paper presents data pertinent to the in-situ stress state and its influence on hydraulic fracturing in the Deep Basin of Alberta. As pan of an extensive research program conducted by Esso Resources Canada Limited in support of its investment in tight gas sands, the vertical distribution of the minimum principal stress was determined over three intervals in two wells in the Deep Basin. Two of these intervals contained zones with sufficient productive promise to warrant hydraulic fracture treatments. These were monitored to a sufficient extent to relate the measured vertical profiles of minimum horizontal stress to the behaviour of the induced hydraulic fractures.

Stress Determinations

Although many methods exist for determining in-situ stresses, the use of hydraulic fracturing is the only practical technique for deep wellbores. During the hydraulic fracturing process a fracture is initiated or reopened by increasing wellbore pressure as fluid is pumped into the well and then closes after pumping stops.