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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.
