The elastic two-phase method

also known as the pseudo-steady-state method or reservoir limit testing method

is a pivotal dynamic technique for estimating the original gas in place（OGIP）in well-controlled scenarios. This method is primarily employed in the initial testing of gas wells and for the OGIP assessment in varied lithologies

fault blocks

and fracture types of gas reservoirs. Since 1994

the pressure squared variant of this method has been recognized in the Chinese national oil and gas industry standards across four editions: SY/T 6098—

1994，SY/T 6098—2000，SY/T 6098—2010，SY/T 6098—2022. This method

based on the pressure squared calculation

offers a robust approximation compared to its pseudo-pressure counterpart. The theoretical derivation of the elastic two-phase equation for pseudo-pressure

originally proposed by AL-HUSSAINY（1966）

and further simplified using WATTENBARGER's（1968）study on the relationship between

μ

g

Z

and

p

allows for representations using both pressure to the first and second powers. Notably

while the pressure to the first power method tends to under-estimate OGIP

the pressure squared method is inclined to over-estimate

as evidenced by practical applications. Currently

there is a lack of substantial literature on the pseudo-pressure elastic two-phase method both domestically and internationally.