The available hydraulic fracturing (HF) techniques for horizontal wells (HW) allow stimulating one or several HW intervals, as well as creating fractures lying at various angles to the horizontal wellbore. The simplest of these methods available to operators is conventional HF; besides, operators may make use of different interval-wise HW stimulation techniques - both with and without packer systems.
During 2006–2007, Rosneft Oil Company performed over 20 HW HF operations. Two HF technologies have been tested: Conventional Frac and multiple-point Hydrajet stimulation. The obtained results demonstrate the efficiency of HF on Rosneft fields, in West Siberian geological conditions: HF has allowed reactivating a number of wells.
The considerable stock of horizontal wells has made it possible to analyze the influence of various factors affecting HF efficiency, such as completion, wellbore orientation, and formation permeability. Several operations were successfully carried out against weak water and gas barriers.
This paper presents Rosneft's experience of HF in horizontal wells and adaptation of conventional HW hydrofrac techniques to West Siberian geological conditions. The article features also some details of HF operations and the results arrived at; HW HF risks and limit conditions for different technologies are defined. Some criteria for HF candidate selection are given.
Horizontal well development has both obvious advantages and disadvantages 1,2,3. Drilling horizontal wells enables a remarkable increase in fluid influx into the well and hence its productivity, thereby higher development rates and recovery efficiency may be achieved. Besides, the use of HWs proves efficient in developing naturally fractured reservoirs, work around coning and problems arising from closely adjoining water- and gas-saturated layers with thin barriers.
In practice, however, the rates projected from HW drilling are frequently not achieved. Given the cost of HWs being on average twice or more the cost of vertical wells, the consequences of such low performance turn out to be far graver. The chief disadvantage of a horizontal well is the risk of having only one productive interval, which deflates severely its efficiency in a field with high cross-section heterogeneity. Even thin shaly barriers can fully isolate some untapped layers from the wellbore. A low vertical permeability of the reservoir is also conducive to reducing HW productivity.
Before the wide spread of controlled drilling, errors in horizontal hole-making were quite frequent, with HWs even placed in some cases in pure clay. When there is no hydrodynamic contact between the wellbore and a productive layer, such errors are fatal. Besides, while drilling HWs, the reservoir medium is exposed to brine for a far longer period than during vertical drilling.