Wells – Processes – Specific propping feature
Reexamination Certificate
1998-12-19
2001-02-27
Suchfield, George (Department: 3672)
Wells
Processes
Specific propping feature
C166S300000, C166S308400, C507S202000, C507S921000
Reexamination Certificate
active
06192985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present Invention relates to method to stimulate the production of hydrocarbons from a subterranean formation. In particular, the present Invention discloses and claims methods to enhance removal of spent fracturing fluid from a fracture deliberately created in the formation, thereby increasing effective fracture length, and thereby increasing hydrocarbon production.
2. Introduction to the Technology
The present Invention relates generally to hydrocarbon (petroleum and natural gas) production from wells drilled in the earth. Obviously, it is desirable to maximize both the rate of flow and the overall capacity of hydrocarbon from the subsurface formation to the surface, where it can be recovered. One set of techniques to do this is referred to as stimulation techniques, and one such technique, “hydraulic fracturing,” is the subject of the present Invention. The rate of flow, or “production” of hydrocarbon from a geologic formation is naturally dependent on numerous factors. One of these factors is the radius of the borehole; as the bore radius increases, the production rate increases, everything else being equal. Another, related to the first, is the flowpaths from the formation to the borehole available to the migrating hydrocarbon.
Drilling a hole in the subsurface is expensive—which limits the number of wells that can be economically drilled—and this expense only generally increases as the size of the hole increases. Additionally, a larger hole creates greater instability to the geologic formation, thus increasing the chances that the formation will shift around the wellbore and therefore damage the wellbore (and at worse collapse). So, while a larger borehole will, in theory, increase hydrocarbon production, it is impractical, and there is a significant downside. Yet, a fracture or large crack within the producing zone of the geologic formation, originating from and radiating out from the wellbore, can actually increase the “effective” (as opposed to “actual”) wellbore radius, thus, the well behaves (in terms of production rate) as if the entire wellbore radius were much larger.
Fracturing (generally speaking, there are two types, acid fracturing and propped fracturing, the latter is of primary interest here) thus refers to methods used to stimulate the production of fluids resident in the subsurface, e.g., oil, natural gas, and brines. Hydraulic fracturing involves literally breaking or fracturing a portion of the surrounding strata, by injecting a specialized fluid into the wellbore directed at the face of the geologic formation at pressures sufficient to initiate and extend a fracture in the formation. More particularly, a fluid is injected through a wellbore; the fluid exits through holes (perforations in the well casing lining the borehole) and is directed against the face of the formation (sometimes wells are completed openhole where no casing and therefore no perforations exist so the fluid is injected through the wellbore and directly to the formation face) at a pressure and flow rate sufficient to overcome the minimum in-situ rock stress (also known as minimum principal stress) and to initiate and/or extend a fracture(s) into the formation. Actually, what is created by this process is not always a single fracture, but a fracture zone, i.e., a zone having multiple fractures, or cracks in the formation, through which hydrocarbon can flow to the wellbore.
In practice, fracturing a well is a highly complex operation performed with precise and exquisite orchestration of equipment, highly skilled engineers and technicians, and powerful integrated computers monitoring rates, pressures, volumes, etc. During a typical fracturing job, large quantities of materials often in excess of a quarter of a million gallons of fluid, will be pumped at high pressures exceeding the minimum principal stress down a well to a location often thousands of feet below the surface.
A typical fracture zone is shown in context, in FIG.
1
. The actual wellbore—or hole in the earth into which pipe is placed through which the hydrocarbon flows up from the hydrocarbon-bearing formation to the surface—is shown at
10
, and the entire fracture zone is shown at
20
. The vertical extent of the hydrocarbon-producing zone is ideally (but not generally) coextensive with the fracture-zone height (by design). These two coextensive zones are shown bounded by
22
and
24
. The fracture is usually created in the producing zone of interest (rather than another geologic zone) because holes or perforations,
26
-
36
, are deliberately created in the well casing beforehand; thus the fracturing fluid flows down (vertically) the wellbore and exits through the perforations. Again, the reservoir does not necessarily represent a singular zone in the subterranean formation, but may, rather represent multiple zones of varying dimensions.
Thus, once the well has been drilled, fractures are often deliberately introduced in the formation, as a means of stimulating production, by increasing the effective wellbore radius. Clearly then, the longer the fracture, the greater the effective wellbore radius. More precisely, wells that have been hydraulically fractured exhibit both radial flow around the wellbore (conventional) and linear flow from the hydrocarbon-bearing formation to the fracture, and further linear flow along the fracture to the wellbore. Therefore, hydraulic fracturing is a common means to stimulate hydrocarbon production in low permeability formations. In addition, fracturing has also been used to stimulate production in high permeability formations. Obviously, if fracturing is desirable in a particular instance, then it is also desirable, generally speaking, to create as large (i.e., long) a fracture zone as possible—e.g., a larger fracture means an enlarged flowpath from the hydrocarbon migrating towards the wellbore and to the surface.
Yet many wells behave as though the fracture length were much shorter because the fracture is contaminated with fracturing fluid (i.e., more particularly, the fluid used to deliver the proppant as well as a fluid used to create the fracture, both of which shall be discussed below). The most difficult portion of the fluid to recover is that retained in the fracture tip—i.e. the distal-most portion of the fracture from the wellbore. Thus, the result of stagnant fracturing fluid in the fracture naturally diminishes the recovery of hydrocarbons. The reasons for this are both simple and complex. Most simply, the presence of fluid in the fracture acts as a barrier to the migration of hydrocarbon from the formation into the fracture. More precisely, the (aqueous-based fluid) saturates the pore spaces of the fracture face, preventing the migration of hydrocarbon into the same pore spaces, i.e., that fluid-saturated zone has low permeability to hydrocarbon.
Indeed, diminished effective fracture length caused by stagnant fluid retained in the fracture tip is perhaps the most significant variable limiting hydrocarbon production (both rate and capacity) from a given well. This is particularly true for low permeability gas reservoirs (approx. <50 millidarcys). The significance of this stagnant fluid on well productivity is evidenced by the empirical observation well known to the skilled reservoir engineer that effective fracture lengths (the true fracture length minus the distal portion of the fracture saturated with fracturing fluid) are generally much less than the true hydraulically-induced fracture length. To achieve an increase in effective fracture length—so that it approaches the true fracture length-therefore involves removing stagnant fracturing fluid from the fracture.
The deliberate removal of fracturing fluid from the fracture is known as “clean-up,” i.e., this term refers to recovering the fluid once the proppant has been delivered to the fracture. The current state-of-the-art method for fracture clean-up involves very simply, pumping or allowing the fluid to flow out of the fracture—thus the fracture fluid
England Kevin W.
Hinkel Jerald J.
Bani-Jamali Maryam
Mitchell Thomas
Nava Robin C.
Schlumberger Technology Corporation
Suchfield George
LandOfFree
Fluids and techniques for maximizing fracture fluid clean-up does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fluids and techniques for maximizing fracture fluid clean-up, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fluids and techniques for maximizing fracture fluid clean-up will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2591618