Abstract：
Long horizontal wells combined with multi-stage hydraulic fracturing make it possible to successfully develop marine
shale. However, the geological characteristics of continental or transitional shale in China are significantly different from those of
marine shale, and the successful techniques used in marine shale cannot be directly applied during the development of continental
or transitional shale. In this work, we proposed that vertical wells can be combined with horizontal wells to develop continental
and transitional shale. The reasons for this technical suggestion include that continental and transitional shale (1) usually contain a
low proportion of brittle minerals, which results in relatively simple fractures created during the multi-stage hydraulic fracturing;
(2) usually contain a high proportion of clay minerals, and the amount of formation damage due to the invasion of fracturing fluid
during the multi-stage hydraulic fracturing is high, compared with formation damage for vertical wells ; (3) usually co-exist with
other types of unconventional gas in other formations vertically (tight gas, coal-bed methane, etc.), and commingling production
together from these unconventional gas layers by using vertical wells can control more reserves. Using the geological and production
data from the Yanchang Petroleum Group in the Ordos basin, China, we analyzed in detail the effects of the aforementioned charac
teristics on the production performance of shale gas wells by using vertical completion and horizontal completion. In addition, four
characterization parameters including vertical reserve index (the ratio of reserves controlled by horizontal wells and that controlled
by vertical wells), brittleness index, formation damage index and silty laminae index were proposed to help the decision makers/
operators to select the optimal well type. The results show that, when the vertical reserve index is smaller than 60
%
, brittleness index
is smaller than 35
%
, formation damage index is larger than 20
%
and silty laminae index is smaller than 10
%
, it is recommended to
use vertical wells to develop the target area and vice versa. Then the criterion was applied to analyze the performance of a horizontal
well drilled in the 7th member of the Yanchang formation and a vertical well drilled in the 1st member of the Shanxi formation. The
proposal challenges the traditional practice that only horizontal wells can develop shale gas commercially, and the advantages of the
combined usage of vertical and horizontal wells to develop continental or transitional shale are highlighted. The proposed strategy
provides a fundamental basis for the development of continental and transitional shale economically and effectively
