Laboratory experiments on 150 x 150 x 250 mm dry blocks of hydrostone have been conducted to examine the mechanisms of hydraulic fracture initiation from horizontal wellbores. Openhole, cased and perforated, and cased and slotted 19 mm diameter wellbores were considered. Wellbores were deviated from the minimum principal stress direction at various angles. Identical confining stresses and fluid injection rate were used throughout the experiments. A few tests were run at a higher injection rate. A single fracture perpendicular to the minimum principal stress direction was initiated and propagated from wellbores with small deviation angles regardless of the wellbore completion. At a higher injection rate, a pair of mutually perpendicular fractures was created in an open hole with deviation. At large deviation angles, a single fracture was initiated along the wellbore axis and propagated as a single fracture for all wellbore completions. At 45 deviation angle, fracture initiation was irregular; however, multiple parallel fractures nearly perpendicular to the minimum principal stress direction were developed and propagated for all wellbore completions. As the deviation angle increased, the fracture initiation pressure decreased. Fracture extension pressure increased as the wellbore deviation increased.
An increased interest in horizontal drilling within the petroleum industry has generated great activity in wide areas since horizontal drilling can provide a substantial economic gain. In naturally-fractured reservoirs, horizontal drilling can provide a communication channel intersecting the pre-existing fractures and fracture stimulation is usually not required. In horizontal wells which require fracture stimulation, the outcome is mixed. In some instances, performance is poor and horizontal drilling is not used. The reason for these less desirable results and accompanying reluctance is possibly the lack of understanding in successfully completing and fracturing horizontal wells. When horizontal wells are deviated from the in-situ principal stress directions, the created fracture plane is not likely to sweep the entire open wellbore section. It is possible to create several parallel fractures in the same wellbore. This can cause difficulty in slurry placement and may result in reduced well productivity. There is a need to find what factors influence fracture initiation and the ultimate fracture communication to the wellbores. Earlier work by Daneshy (1973) discussed the nature of fracturing inclined wellbores. The approach involved theoretical and experimental work. Experiments were conducted on moderately inclined wellbores with open hole completion. Field evaluation of inclined wellbore fracturing was performed by Kim et al. (1989). They confirmed multiple fractures in a single well and limited communication between the wellbore and created fracture. Site-specific problems in fracturing inclined wells were studied by Veeken et al. (1989), and Hallam and Last (1990) with laboratory model tests. El Rabaa (1989) experimentally studied fracturing horizontal wells with emphasis on the effect of perforation interval on hydraulic fractures. Systematic studies are lacking. As a first step to approach the complex task on fracturing horizontal wells, problems associated with fracture initiation have been investigated in these experiments. Anticipated problems and their consequences in completing and fracturing horizontal wells are also addressed.
