Methods of stimulating and producing multiple stratified reservoirs

Methods of stimulating and producing multiple stratified hydrocarbon reservoirs having numerous separate reservoir compartments are provided. The methods basically comprise the steps of drilling a first well bore into a lower part of the reservoir having a horizontal portion which intersects a previously drilled second well bore therein. At least one fracture is formed extending into two or more reservoir compartments from the horizontal portion of the first well bore for conducted hydrocarbons in the reservoir into the horizontal portion of the first well bore from where the hydrocarbons flow into the second well bore and are withdrawn.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to methods of stimulating and producing 
multiple stratified low permeability hydrocarbon reservoirs having 
numerous separate reservoir compartments. 
2. Description of the Prior Art 
Field areas containing multiple stratified or laminated hydrocarbon bearing 
formations exist in some parts of the world. Such field areas are 
comprised of a large number of sandstone or other permeable rock layers 
containing hydrocarbons that are encased or separated by shale or other 
relatively impermeable rock layers of varying thickness. In addition, the 
sandstone layers often do not extend in a homogeneous fashion over an 
extensive area due to lateral stratigraphic variations and structural 
trapping features such as sealing faults. This lateral stratigraphic 
variation and structural trapping coupled with the presence of impermeable 
rock layers create numerous separate reservoir compartments of varying 
size over a relatively large vertical laminated sequence and field area. 
In many field areas, these reservoir compartments contain large quantities 
of hydrocarbons. 
The production of hydrocarbons from multiple stratified hydrocarbon 
reservoirs has heretofore been a low economic return venture for oil and 
gas exploitation companies even when significant oil and gas has been 
confirmed to be in place. The problem is that the hydrocarbons are 
contained in numerous relatively small reservoir compartments, many of 
which cannot be practically or economically penetrated by well bores. The 
problem is further complicated by the fact that the reservoir formations 
containing the hydrocarbons have relatively low permeabilities. 
Heretofore, attempts have been made to produce the low permeability 
reservoir compartments of multiple stratified reservoirs by way of 
hydraulic fracture stimulated wells. These well stimulation treatments 
involve the injection of viscous fracturing fluids into subterranean 
formations at rates and pressures sufficient to fracture the formations. 
Proppant material, such as sized sand, is mixed with the fracturing fluid 
to keep the created fractures open after the fracturing process is 
concluded. In most cases, the fractures formed in the stratified 
hydrocarbon bearing formations are vertically oriented and extend 
outwardly from the well bore in a direction perpendicular to the least 
principal formation stress in the horizontal plane. 
Due to variances and uncertainties related to rock mechanical properties 
and formation pore pressures in the sandstone reservoir compartments and 
the shale intervals that encase the sandstone, attempts to propagate 
fractures through the compartments has yielded unpredictable and often 
poor results using prior art practices. Furthermore, problems have 
historically been experienced in propping shale intervals located between 
more permeable sandstone formations due in part to the lack of fracture 
fluid leak off adjacent to the shale intervals. Soon after fracturing 
fluid injection operations are concluded and during fracture closure, the 
fracturing fluid tends to migrate toward the sandstone formation intervals 
as the fluid portion of the fracturing fluid leaks off causing relatively 
low proppant concentration adjacent to the shale intervals. This fracture 
width pinching phenomena is often compounded by increased proppant 
embedment adjacent to the shale intervals. The resulting poor conductivity 
of the propped fracture adjacent to the shale intervals impedes the 
desired commingling of the separate reservoir compartments into one well. 
In crude oil bearing multiple stratified formations, the highly 
compartmentalized reservoirs are typically solution gas driven whereby the 
predominant reservoir energy causing the crude oil to move toward 
production wells completed in the reservoirs is the expansion of the gas 
in solution with the crude oil under pressure. Typically, after a 
relatively small percentage of the oil in the reservoir has been produced, 
the reservoir pressure declines to a level allowing the gas to break out 
of solution from the crude oil and become free natural gas in the 
reservoir. Because the viscosity of natural gas is much less than the 
viscosity of liquid crude oil, the natural gas bypasses the crude oil as 
it preferentially flows through the reservoir toward the production wells. 
This is detrimental to the efficient production of the more valuable crude 
oil because of the loss of the gas drive. Gas breaking out of solution 
with the crude oil in the reservoir also adversely effects the relative 
formation permeability to the crude oil as is well known by those skilled 
in the art of reservoir engineering. 
The recovery efficiency of solution gas drive oil reservoirs is relatively 
low unless secondary or enhanced oil recovery processes are employed, 
i.e., unless certain gases, steam, chemicals and/or water are injected 
from specially equipped wells completed at strategic locations in the 
reservoir to flood, sweep or otherwise drive the crude oil toward the 
production wells and/or to maintain reservoir pressure at a high enough 
level whereby the gas remains in solution with the crude oil. 
Unfortunately, due to the relatively small size of each reservoir 
compartment and the heterogeneous nature of the hydrocarbon containing 
formations in most multiple stratified reservoirs, secondary recovery and 
enhanced oil recovery processes have not been effective using prior art 
methods. 
Thus, there is a need for improved methods of stimulating and producing 
multiple stratified hydrocarbon reservoirs whereby effective propped 
fractures are created in the formations which commingle various reservoir 
compartments and allow the reservoirs to be efficiently produced. Further, 
in cases where such stratified reservoirs primarily contain crude oil, 
there is a need for such methods whereby the crude oil is produced by 
gravity drainage and solution gas expansion drive in combination with 
enhanced oil recovery processes enabling a larger percentage of the oil 
originally in place to be recovered at a lower cost per barrel of oil 
produced. 
SUMMARY OF THE INVENTION 
The present invention provides improved methods of stimulating and 
producing wells in a multiple stratified hydrocarbon reservoir having 
numerous separate reservoir compartments which meet the needs described 
above and overcome the deficiencies of the prior art. The methods are 
basically comprised of the steps of drilling a first well bore into a 
lower part of the stratified reservoir having a horizontal, preferably 
downwardly sloped portion which intersects a previously drilled second 
substantially vertical well bore therein. A third well bore is optionally 
drilled into an upper part of the reservoir having at least one horizontal 
portion positioned substantially over the horizontal portion of the first 
well bore. One or more vertically oriented propped fractures are then 
formed which extend upwardly through two or more of the reservoir 
compartments from the horizontal portion of the first well bore thereby 
commingling the reservoir compartments. When the third well bore is 
drilled, each fracture extends between the horizontal portion of the first 
well bore and a horizontal portion of the third well bore. Hydrocarbons in 
the fracture communicated reservoir compartments flow into the horizontal 
portion of the first well bore by way of the fractures and then into the 
second well bore from where the hydrocarbons are withdrawn. 
In order to increase the flow of hydrocarbons into the second well bore, 
natural gas from the reservoir can be produced from the third well bore, 
chemicals to facilitate oil production can be injected into the reservoir 
by way of the third well bore and/or a compressible fluid such as natural 
gas, carbon dioxide, steam or the like can be injected into the reservoir 
by way of the third well bore. 
It is, therefore, a general object of the present invention to provide 
improved methods of stimulating and producing multiple stratified 
reservoirs.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention provides improved methods of stimulating and 
producing multiple stratified low permeability hydrocarbon reservoirs. As 
mentioned above, such reservoirs have been fracture stimulated and 
produced heretofore, but the fracture stimulation treatments have been 
only marginally successful due to insufficient fracture extension and lack 
of adequate propping. The improved methods of this invention enable 
hydrocarbons from the hydrocarbon containing intervals to be co-mingled 
and produced in a manner whereby gravity drainage of the reservoir is 
optimized. In addition, the drainage of liquid hydrocarbons from the 
reservoir can be increased by employing certain enhanced oil recovery 
processes whereby natural gas, carbon dioxide, steam or other compressible 
fluid is injected into the top portion of the reservoir. The injected 
fluid provides a pressure drive that increases the flow of the liquid 
hydrocarbons to the bottom of the reservoir from where they are withdrawn. 
Furthermore, compressible fluids injected, with or without staged 
injection of incompressible fluids, into one or more additional vertical 
well bores selectively drilled at strategic locations relative to one or 
more production well bores can be used to provide a flood or sweep or 
otherwise drive the hydrocarbons laterally to the production well bores as 
will be further described hereinbelow. In certain cases, the injection of 
fluids for enhanced oil recovery can also serve to maintain reservoir 
pressure at a level sufficient to minimize the adverse effects of 
dissolved natural gas breaking out of solution with the crude oil in the 
reservoir. 
As is customary in developing a technique for producing hydrocarbons from a 
reservoir, data is acquired and analyzed from new and existing wells in 
the multiple stratified reservoir to be stimulated and produced to 
determine the mechanical properties of the multiple permeable and 
impermeable formations making up the reservoir. In such a reservoir, a 
hydraulic fracture stimulation treatment performed in a well bore will 
generally induce a single fracture plane that is vertically oriented and 
perpendicular to the direction of the least principle horizontal stress in 
the reservoir. The data and information acquired including the direction 
of the least principle horizontal stress is utilized in performing the 
methods of this invention as described hereinbelow. 
The term "horizontal" used herein when referring to a well bore or a 
portion of a well bore means that the well bore or portion thereof is 
positioned in the range of from about 30.degree. to about 90.degree. from 
vertical. The term "verticall" used herein when referring to a well bore 
or portion thereof means that the well bore or portion is positioned in 
the range of from 0.degree. to about 30.degree. from vertical. 
The methods of this invention are basically comprised of the following 
steps. A first well bore is drilled into a lower part of the multiple 
stratified reservoir to be stimulated and produced. The first well bore 
includes a horizontal portion which is preferably sloped downwardly toward 
and intersects a previously drilled vertical second well bore in the 
reservoir. The intersection of the first well bore with the second well 
bore is preferably above the bottom of the second well bore whereby a sump 
is formed into which liquid hydrocarbons flow and from which they are 
pumped to the surface. An optional third well bore is preferably drilled 
into an upper part of the reservoir which includes at least one horizontal 
portion. One or more fractures are then formed in the reservoir extending 
into two or more separate reservoir compartments between the horizontal 
portions of the first and third well bores whereby hydrocarbons in the 
reservoir are co-mingled and flow into the horizontal portion of the first 
well bore by way of the fractures formed in the reservoir. In order to 
increase the flow of liquid hydrocarbons into the first well bore, 
fracture clean out chemicals and the like and/or a compressible fluid such 
as natural gas, carbon dioxide or steam can optionally be injected into 
the upper portion of the reservoir by way of the horizontal portion of the 
third well bore. 
The liquid hydrocarbons and any associated formation water from the 
commingled reservoir compartments flow by gravity and pressure drive into 
the first well bore and into the sump of the second well bore from where 
they flow or are pumped to the surface. Generally, the liquid hydrocarbons 
and water are pumped to the surface by a downhole mechanical pump 
positioned in the sump below the intersection with the first well bore. In 
certain cases, the sump can be configured for subsurface separation of the 
crude oil and formation water enabling the formation water to be 
re-injected downhole rather than lifted to surface with the crude oil. as 
mentioned, the horizontal portion of the first well bore is preferably 
drilled at an angle from vertical of approximately 80.degree. to produce a 
downward slope into the sump which promotes gravity flow, minimizes solids 
buildup, and minimizes well bore friction. 
As also mentioned above, in order to increase the flow of liquid 
hydrocarbons into the sump, a compressible fluid such as natural gas, 
carbon dioxide, steam or the like can be injected into the commingled 
reservoir compartments by way of the third well bore. As the gas fills the 
uppermost reservoir compartments, it will cause the liquid hydrocarbons to 
migrate to underlying reservoir compartments and into the first well bore 
by way of the fractures. Furthermore, additional vertical injection well 
bores can be drilled in strategic locations relative to the multiple 
fractures in the reservoir for the injection of gases, water and/or 
alternating stages of gas and water into the various reservoir 
compartments. The injection wells can also be fractured to produce 
fractures parallel to the fractures created from the first well bore, but 
offset some distance to allow hydrocarbon liquids to be flooded, swept or 
otherwise driven toward the first well bore fractures. Selective flow 
control devices can be installed in the injection wells to allow injection 
fluids to target specific reservoir compartments according to injection 
schedules designed to allow optimized reservoir production. 
As is well understood by those skilled in the art, when the horizontal 
portion of the first well bore which intersects the second well bore is 
oriented in a direction approximately parallel to the direction of the 
least principle horizontal stress in the reservoir, the vertical fractures 
formed are oriented in directions approximately perpendicular or 
transverse to the well bore. So that the transversely oriented fractures 
will intersect the third well bore, the horizontal portion of the third 
well bore is drilled in a direction substantially parallel to the 
horizontal portion of the first well bore. One or more secondary laterals 
may be drilled from the horizontal portion of the third well bore in a 
configuration to maximize the probability that the vertical fractures 
formed will intersect the third well bore. The horizontal portion or 
portions of the third well bore can be completed open hole, with a slotted 
liner or in other suitable fashions known to those skilled in the art. A 
pressure/temperature sensor can be installed in the third well bore so 
that when the third well bore is intersected by a fracture being formed, 
the fracturing fluid entering the third well bore will be sensed by the 
pressure/temperature sensor and the fluid will flow out of the reservoir 
by way of the third well bore. 
In some applications of the present invention, it is preferable to form a 
single vertical fracture extending from the horizontal portion of the 
first well bore along its length rather than forming a plurality of 
vertical fractures oriented transversely to the horizontal portion. In 
such an application, the horizontal portion of the first well bore is 
formed in a direction substantially perpendicular to the direction of 
least principle horizontal stress in the reservoir, and the horizontal 
portion or portions of the third well bore are formed in directions 
transverse to the direction of the horizontal portion of the first well 
bore. The resulting single fracture extends upwardly from the horizontal 
portion of the first well bore along the length of the well bore and 
intersects the horizontal portion or portions of the third well bore 
extending transversely to the direction of the fracture. 
When a plurality of spaced fractures oriented transversely to the 
horizontal portion of the first well bore are required, they can be formed 
utilizing techniques known to those skilled in the art. A presently 
preferred such technique is to complete the first well bore with cemented 
casing in the non-horizontal portion thereof and a cemented liner in the 
horizontal portion thereof. A retrievable bridge plug is set in the liner 
to isolate the liner from the second well bore, and a first opening is cut 
in the liner which faces the horizontal portion or portions of the third 
well bore. The opening can be cut in the liner utilizing various 
techniques, e.g., an abrasive liquid slurry jetting technique. After the 
first opening has been formed, a hydraulic fracturing fluid is pumped into 
the liner by way of the first well bore and through the first opening at a 
rate and pressure sufficient to create a fracture and extend it until it 
intersects a horizontal portion of the third well bore. 
The fracturing fluid utilized can be any of the viscous fracturing fluids 
known to those skilled in the art which include suspended proppant 
material so that when completed the fracture will be propped open. 
Preferably, the fracturing fluid includes a high concentration of curable 
resin coated proppant and the fracturing fluid is designed to produce a 
tip screen-out after the fracture has intersected the horizontal portion 
or portions of the third well bore. In a tip screen-out, the proppant is 
caused to pack-off against the tip of the fracture causing further 
fracture extension to stop. After initiating the tip screen-out, the 
fracture pressure increases as the fracture width increases. The fracture 
is packed with a relatively high concentration of proppant as continuous 
leak-off occurs through the walls of the induced fracture. Upon the curing 
of the resin coated proppant, a highly permeable fracture is formed that 
effectively co-mingles the compartments of the reservoir through or into 
which the fracture extends. 
After the first fracture has been completed, the bridge plug is moved and 
reset so that the liner is isolated from the first fracture as well as the 
second well bore. A second opening is cut into the liner spaced a distance 
from the first opening and additional hydraulic fracturing fluid is pumped 
into the liner by way of the second well bore. The fracturing fluid flows 
through the second opening at a rate and pressure sufficient to create a 
second fracture and extend it to a horizontal portion of the third well 
bore. The process of isolating the liner, cutting an opening therein and 
pumping fracturing fluid is repeated to produce additional spaced 
fractures between the horizontal portions of the first and third well 
bores along the length of the horizontal portion of the first well bore. 
When the horizontal portion of the first well bore extends in a direction 
substantially perpendicular to the direction of least principle horizontal 
stress in the reservoir and a single fracture extending therefrom is to be 
formed, the liner is isolated from the second well bore with a retrievable 
bridge plug as described above. A plurality of upwardly facing openings 
are formed in spaced relationship along the length of the liner. A 
hydraulic fracturing fluid containing proppant material is then pumped 
into the liner and through the spaced openings at a rate and pressure 
sufficient to create a fracture and extend it to the horizontal portion or 
portions of the third well bore. The fracture is packed with proppant as 
described above to thereby provide a permeable conduit through which 
hydrocarbons in the reservoir can flow into the horizontal portion of the 
first well bore. 
As will now be understood, the fracture or fractures formed between the 
horizontal portions of the first and third well bores provide one or more 
flow passages through at least two and preferably more of the separate 
compartments of the reservoir whereby hydrocarbons co-mingle and flow to 
the horizontal portion of the first well bore. The hydrocarbons flow 
through the horizontal portion of the first well bore to the second well 
bore from which the hydrocarbons are withdrawn. As will also be 
understood, additional sets of interconnected first, second and third well 
bores and injection wells can be drilled throughout the multiple 
stratified reservoir field area to thereby simultaneously produce 
hydrocarbons from the entire reservoir. 
In order to further illustrate the methods of this invention, drawings 
showing a multiple stratified reservoir with well bores and fractures 
formed therein in accordance with the methods of this invention are 
provided. Referring to the drawings, and particularly to FIG. 1, a 
multiple stratified reservoir is illustrated and generally designated by 
the numeral 10. As described above, the reservoir 10 is comprised of 
hydrocarbon containing sandstone layers 12 having layers of shale or other 
rock 14 therebetween. 
A first well bore 16 is drilled into a sandstone layer 12 in a lower part 
of the reservoir 10 and extended horizontally whereby the horizontal 
portion 17 intersects a previously drilled second well bore 18 at a point 
31 whereby a sump portion 19 of the second well bore extends below the 
intersection. As mentioned above, the horizontal portion 17 of the first 
well bore 16 is sloped downwardly toward the second well bore 18. As 
described above, an optional third well bore 20 is preferably drilled into 
an upper part of the reservoir 10 which also includes a horizontal portion 
21. In the embodiment illustrated in the drawings, the horizontal portion 
17 of the well bore 16 is drilled in a direction substantially parallel to 
the direction of least principle horizontal stress in the reservoir 
whereby when spaced fractures 22 (shown in dashed lines) are formed, they 
are substantially perpendicular to the horizontal portion of the first 
well bore 16. 
Referring now to FIGS. 2 and 3, the well bores 16, 18, and 20 and the 
fractures 22 are shown in greater detail. The first well bore 16 includes 
casing 24 cemented in the non-horizontal portion thereof and a liner 26 
cemented in the horizontal portion thereof. The liner 26 includes a 
plurality of spaced openings 28 cut therein with the spaced fractures 22 
extending between the openings 28 in the liner 26 and the horizontal 
portion 21 of the third well bore 20. The second and third well bores 18 
and 20 are shown having casing 30 and 32, respectively, cemented in the 
non-horizontal portions thereof. The horizontal portion of the third well 
bore 20 is completed open-hole. A liquid hydrocarbon pump 34 is disposed 
in the sump portion 19 of the second well bore 18. 
In order to illustrate the methods of the invention still further, the 
following example is given. 
EXAMPLE 
Referring again to the drawings, a multiple stratified reservoir 10 
comprised of low permeability heterogenous sandstone layers 12 separated 
by shale layers 14 exists in an interval of about 1000 feet. The 
permeabilities of the sandstone layers 12 to air range from less than 1 md 
to approximately 50 md with an average of about 8 md. The porosities of 
the sandstone layers 12 range from about 12% to about 16%. All of the 
sandstone layers 12 contain oil with connate water saturations of 
approximately 30% and solution gas drives. The gas to oil ratios of the 
hydrocarbons produced from the layers range from about 500 to about 1000 
standard cubic feet per barrel. The gravity of the crude oil is between 
about 22.degree. and 24.degree. API. 
A first well bore 16 is drilled having a horizontal portion near the bottom 
of the reservoir 10 which is about 3500 feet long. The horizontal portion 
of the first well bore 16 extends in a direction substantially parallel to 
the direction of least principle horizontal stress in the reservoir 10 and 
intersects a previously drilled vertical second well bore 18. A third well 
bore 20 is drilled into an upper part of the reservoir 10 having a 
horizontal portion above and substantially parallel to the horizontal 
portion of the first well bore 16. Three transverse vertical fractures 22 
spaced about 500 feet from each other are formed between the horizontal 
portions of the first and third well bores 16 and 20. The three fractures 
are propped and have radiuses from the horizontal portion of the first 
well bore of about 400 feet. The drainage area of the well bore and 
fracture system is about 155 acres and the average net effective pay depth 
is about 235 feet. The oil initially in place is about 20,640,000 barrels, 
25% or more of which will be recovered by the methods of the present 
invention. 
Thus, the present invention is well adapted to carryout the objects and 
attain the ends and advantages mentioned as well as those which are 
inherent therein. While numerous changes may be made by those skilled in 
the art, such changes are encompassed within the spirit of this invention 
as defined by the appended claims.