Risk assessment-based design method for deep complex formation wellbore structure

Abstract

A risk assessment-based design method for a deep complex formation wellbore structure includes: (1) preliminarily determining casing layers and setting depths; (2) calculating to obtain the risk coefficients of each layer of casing; (3) analyzing and coordinating, according to the principle that a shallow casing shares more risks and a deep casing shares less risks, the risks of each layer of casing: determining whether the risk coefficients of each layer of casing are greater than a safety threshold value K; checking the setting depth: if the safety coefficient of an ith-layer casing satisfies RNi>K, selecting a casing layer with the minimum safety coefficient from upper casing layers, and deepening the setting depth h of the casing layer; and (4) repeating the steps (2) to (3) until the casing risk coefficients of each layer of casing are less than the safety threshold value K.

What is claimed: 
     
         1 . A risk assessment-based drilling method with a deep complex formation wellbore structure, wherein the deep complex formation wellbore structure is controlled by a device that comprises a computer-readable device and an instruction, and the device executes a processor for performing the following steps:
 (i) generating, by the processor, casing layers and a setting depth;   (ii) creating a probabilistic distribution of formation pressure and prediction accuracy, by the processor, according to a prediction error of formation pressure and a well depth wherein the prediction error of formation pressure is correlated to the well depth;    wherein a probabilistic distribution of the prediction error of formation pressure is created, by the processor, based on a standard deviation and the prediction accuracy;    creating, by the processor, a cumulative probability corresponding to the formation pressure by integrating the probabilistic distribution of the prediction error of formation pressure wherein the cumulative probability is greater than the prediction error of formation pressure;   
       creating, by the processor, a probabilistic distribution of wellbore structure design coefficient based on a threshold value of a safety coefficient and a standard deviation of the threshold value of the safety coefficient;
  generating, by the processor, a distribution interval of the threshold value of the safety coefficient with a credibility and the cumulative probability; wherein the probabilistic distribution of wellbore structure design coefficient is greater than the cumulative probability, any specific value of the cumulative probability is within the range of the probabilistic distribution of wellbore structure design coefficient; 
  creating, by the processor, a downhole engineering risk at a specific well depth according to a pressure balance relationship which is created by the processor with a kick risk; the kick risk is resulted from a risk of lost circulation, an equivalent density of drilling fluid, a formation pore pressure, a minimum limit of the formation pore pressure, and the depth of a last casing shoe; wherein a wellbore pressure is less than the sum of the formation pore pressure and the probabilistic distribution of wellbore structure design coefficient; 
 
       creating, by processor, a risk coefficient by integrating the downhole engineering risk, wherein a specific downhole engineering risk is within a specific well structure scheme;
 (iii) selecting, by the processor, a specific risk coefficient of each layer of casing; wherein the specific risk coefficient of each layer of casing is greater than the threshold value of the safety coefficient; 
 selecting, by the processor, a specific setting depth wherein the safety coefficient is greater than the threshold value of the safety coefficient, selecting a casing layer with a minimum safety coefficient from the casing layers, and deepening the setting depth of the casing layer; 
 (iv) repeating (ii) to (iii) until the risk coefficient of each layer of casing are less than the threshold value of the safety coefficient; and 
 (v) positioning the deep complex formation wellbore structure for drilling based on the risk coefficient which provides data for overall planning and quantitative optimization of casing running depth at each level before drilling. 
 
     
     
         2 . The risk assessment-based drilling method according to  claim 1 , wherein the method in the step (i) further comprises:
 (a) measuring a geological setting position;   (b) creating, by the processor, a safety pressure window based on prediction results of formation pore pressure, formation fracture pressure and formation collapse pressure before drilling and a pressure balance relationship of an open hole section; and   (c) generating, by the processor, the casing layers and the setting depth based on the geological setting position, the safety pressure window and a regional wellbore structure design coefficient.