| CPC E21B 49/006 (2013.01) [E21B 47/06 (2013.01)] | 1 Claim |
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1. A method for determining well wall collapse of a single well for a fractured-vuggy reservoir, comprising:
S1, treating the single well of the fractured-vuggy reservoir as an isolated reservoir, wherein water influx exists in a production process;
S2, treating a water body for the fractured-vuggy reservoir as a limited water body, and calculating a water influx rate;
S3, substituting a formula of the water influx rate into a material balance equation to obtain a formula for a formation pressure of the reservoir;
S4, calculating a change of an oil productivity index without considering the well wall collapse;
S5, determining the well wall collapse through the change of the oil productivity index, and establishing an optimizing model for obtaining the oil productivity index;
S6, calculating the formation pressure at any time in production through step S3;
S7, calculating the oil productivity index at any time through the formation pressure at any time in production, and establishing an optimal objective function; and
S8, calculating a difference between the obtained oil productivity index at any time and the theoretically calculated oil productivity index at any time, and determining whether the difference meets an accuracy requirement; if so, outputting a target value; otherwise returning to step S6 until the difference meets the accuracy requirement, outputting the change of the oil productivity index, and determining whether the well wall collapses according to the change,
wherein in step S1, an exploitation of the fractured-vuggy reservoir satisfies a material balance theory:
 where Np is cumulative oil production; Wp is cumulative water production; Winj is cumulative water injection; We is cumulative water influx; N is a controlled reserve of the single well; Bo is a crude oil volume coefficient; Bw is a formation water volume coefficient; Boi is a crude oil volume coefficient under an original condition; ceff is an effective compressibility coefficient of the reservoir; pi is an original formation pressure of the reservoir; and p is a formation pressure of the reservoir;
wherein in step S2, the water influx rate is calculated by following formula:
 where Vw is a volume of the water body; and cw is a compressibility coefficient of water;
wherein in step S3, a formula of the water influx rate is substituted into a material balance equation to obtain a formation pressure of the reservoir:
 wherein in step S4, first, a flowing pressure and a production or productivity test data at a time of a highest production are obtained from current production data which are denoted as (qomax, pwfmin) in which according to the production of the single well:
 where
 is a constant, which is denoted as:
 and during the well wall collapse, a permeability around the well decreases, or a water content increases, the oil productivity index decreases, and a production formula is written as:
 or written as
 Jo is rewritten as
 a ratio formula of Jo to Joi is obtained:
 the oil productivity index is calculated by the following formula:
 where qomax represents a maximum oil production in the production process; pwfmin represents a minimum flow pressure corresponding to qomax; qo represents the oil production in the production process; pwf represents a flow pressure corresponding to qo; β represents a stress sensitivity index corresponding to collapse; kro represents a relative permeability of an oil phase; μo represents a viscosity of oil corresponding to the formation pressure p; μoi represents a viscosity of oil corresponding to an initial formation pressure pi; and e represents an exponential function;
wherein in step S5, Jo at any time is obtained by obtaining the formation pressure, and the well wall collapse is determined through a change of Jo; and
wherein in step S7, the oil productivity index at any time is calculated through the formation pressure at any time in production:
 the theoretically calculated oil productivity index at any time is obtained from the ratio formula of Jo to Joi;
 at this time, the optimal objective function is established:
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