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Abstract
Hydraulic fracturing is a stimulation technique successfully applied in
formations throughout the world to increase production rates and enhance
hydrocarbon recovery. The process involves creating a crack by pumping fluids
at pressures above formation fracturing pressures, and then filling the crack
with proppant to create a high conductivity connection to a large formation
area. Hydraulic fracturing stimulates production by overcoming restrictions
imposed by formation permeability, drilling and completion damage,
production-induced damage and, an incomplete reservoir connection across
laminated intervals. The process has been applied to a large scale in many
Central and Southern California fields to enable economic development and
reasonable hydrocarbon recovery. Example formations include the Belridge
Diatomite, Stevens Sands, Etchegoin, Antelope shale, McLure shale, McDonald
shale, Point of Rocks sands, Kreyenhagen shale, Ranger sands, the UP Ford
shale, and the Monterey shale. Despite the routine application of fracturing in
many fields, there has been very little fracturing experience in the
gas-producing formations of Northern California. Example formations such as the
Martinez, Forbes, Winters, and the K-1 are generally laminated sand intervals
with low to moderate permeability (less than 1 mD and up to 10 mD), that are
easily damaged by completion and production operations. Despite the hydraulic
fracturing potential for stimulating production rates, improving gas recovery,
and increasing reserves by extending the economic development area, it has been
only sparingly employed. General formation properties are reviewed – what are
the implications for hydraulic fracture potential, treatment design and
placement challenges? Several treatments are reviewed to provide examples of
fracture treatment behavior and response. Based on the initial experience and
formation properties, it is believed that hydraulic fracturing has a
significant potential in many Northern California gas reservoirs.
Introduction
Conventional development of Northern California gas fields has been for the
most part limited to non-stimulated vertical well drilling of intervals with
low to moderate permeability and highly laminated beds. The nature of the
formations and the geology of the basin lend itself to realizing greater
recovery potential with the use of fracturing stimulation technique. Hydraulic
fracturing is believed to have a significant positive impact potential – it can
provide production acceleration from conventional reservoirs, and add new
reserves from unconventional or problematic intervals. It would also provide
similar benefits from interval damage by drilling, completion, and production
operations.
The flow and reservoir deliverability potential as described by Darcy’s law
(shown below for radial gas flow using average reservoir pressure) is directly
proportional to reservoir properties such as formation permeability, zone
height, reservoir pressure and pressure drawdown. On the inverse side,
production rates and reserve recovery can be limited due to the presence of
skin acting as a choke to the full production potential and limited reservoir
drainage area.
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