Table 1
Parameters value range for various reservoirs
Parameters
Reservoir Type
Good
Bad
Ugly
Depth to top of reservoir, m
0-4580
2100-6100
3050-9150
AHFP upper limit position, m
910-3050
910-3660
2440-*
Reservoir temperature, °С
21-121
121-149
149-*
Normal temperature in the depth to top of
reservoir, °С
21-91
93-138
138-*
Depth earth pressure gradient, MPа/m
0,0181-0,0226
0,0204-0,0226 0,0204-0,0271
Estimation of capitalized value of well
drilling, completion and commissioning (С)
low
high
really high
Estimation of net capitalized profit during
production (К)
high
low or average
really low
Commercial factor
really high
average or low
catastrophic
* - Parameters range that are subject to definition are not available
“Good” reservoirs are oil and gas formations the development of which is commercially
profitable.
“Bad” reservoir doesn’t have some positive features of “good” one. However, it still keeps a
commercially profitable quality, and it will be operated if the well has been duly drilled, completed
by drilling and managed.
Drilling for “bad” reservoirs and their further operation are related to raised risk, significant
costs as well as getting over technical and process difficulties.
“Ugly” reservoir doesn’t have major features of “good” one and it never provides inflow to
well of products in commercial sizes.
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As the main parameter for oil and gas reservoirs’ classification in works /1, 2/ there has been
used a depth of AHFP zone. In accordance with this classification the reservoirs of productive
formations may be considered as follows:
•
“good” – in depth up to approximately 600 m below upper limit of AHFP zone;
•
“bad” – in depth up to 600-1500 m below upper limit of AHFP zone;
•
“ugly” – in depth more than 1500 m below upper limit of AHFP zone.
For classification of reservoirs of Fergana area with hydrocarbon potential we have plotted
the formation pressure related to producing depth for all productive formations. Given the above we
can conclude that probability of discovery within Fergana Valley of oil and gas deposits with “good”
reservoirs is limited to depth of 2600 m, “bad” – 2600-3500 m, and “ugly” – more than 3500 m.
Analysis and generalization of available expertise for development of oil and gas fields within
Fergana depression shows that the classification of reservoir types mainly is confirmed.
It’s necessary to note that if for reservoir types classification there will be used just a plot of
formation pressure related to depth of site, it’s possible to find some exceptions from established
limits. Thus, these exceptions are connected with not only non-standard performance of container
rocks of deep-seated oil deposits in AHFP conditions but also with successful development of some
sites located in depth where “bad” and “ugly” reservoirs are available.
As an example, we will address to development expertise of oil deposit of horizon VII in
Ravat field. The specified oil deposit located in the top of alay layers of Paleogene (depth of 3500 m)
consists of light grey and grey pelitomorphic, as well as arenaceous lime stones with light grey
sandstone band and grey green shale streak.
Development well spacing density is on the average 8.5 ac per well. Field drilling out period
proceeding for 10 years has been accompanied by increasing of annual oil extraction, as well as
significant rate of formation pressure drop. Further, notwithstanding producing well stock extension
at 30% the annual oil production sharp reduction is available. After 14 years of development an
observed decreasing of the rate of annual oil production drop has been related to introduction of a
water flooding system. Despite of the producing well stock decreasing at almost 40% this trend has
been available over the past years of development.
An efficiency of inside profile water flooding is clearly shown in the dynamics of formation
pressure that by the commence of water injection has been decreased from virgin pressure of 30.2
MPa (abnormality rate is 0.86) to 13.6 MPa. Over 10 years of water flooding there have been achieved
100% compensation of fluid extraction by water injection that in turn has resulted in formation
pressure increasing up to 22.6 MPa. After that the rate of annual production considerably has been
decreased.
An execution of the project document main statements during development of oil deposit of
horizon VII in Ravat field has allowed to achieve current extraction rate of 0.15. An anticipated design
value of oil extraction final rate is 0.222. It’s more higher than in the sites with “bad” reservoirs
developed in a natural mode and with more rare infrequent well pattern.
At the same time in a depth with “bad” and “ugly” reservoirs location there are available more
than 10 fields the development efficiency of which is too low (expected oil extraction final rate
doesn’t increase 0.10) or their development in production quantities can not be implemented through
existing technology.
Gumkhana field is one of these sites. Its discovered oil deposits are related to deposition of
light pink suit of Neogene presented mainly as sand-shale incompetent differences with sufficient salt
content. Since 1968 there have been drilled 12 wildcats in the fields with average producing depth of
4750 m and a rate of formation pressure abnormality is approximately 2.
Among them seven wells (# 2, 3, 4, 6, 7, 9, 10) have been producing ones. However, just well
#2 has been passed for operation, as well as with insufficient rate of flow of both oil and fluid. The
purpose of abandonment of other wells is emergency particularly connected with carry up of much
sand from productive depositions. It’s recommended to utilize the proposed classification of reservoir
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types selection for the fields conditions of Fergana depression in the well drilling and site
development. For increasing field development efficiency in depth of “bad” and “ugly” reservoirs it’s
required more advanced specialized technologies that keep stability and wall-building properties of
container rocks during filtration of formation fluids.
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