A Study Based On Numerical Simulation For Resistivity Correction In Mud-invasion Reservoir

The problem about mud intrusion of hydrocarbon reservoir exists in many of the processes of oil and gas field exploration and development. It will not only pollutes reservoir and reduces productivity, but it also causes log response characteristic of reservoir distortion to lead to the mispredication of oil and water layer. Foreign experts and scholars have realized the importance of invasion correction and did a lot of work. It has produced a marked effect but is not perfected. So evaluate parameters in pollution reservoir is still a difficulty for interpreters.This paper is based on finite element theory, has proceed forward numerical modeling of lateral log, combined with Jilin oilfield production and carried out the following research:(1) Basic understanding of Oil fieldYtong basin is a Cenozoic strike-slip-stretch faulted basin located between Changchun and Jilin City of Jilin province. As a major increased production block in Jilin oil field, mud invasion is very serious in this block. Meanwhile, Oil, gas, water logging response characteristics are complex and the contrast of resistivity is low. Original salinity is about6000ppm.This block exists both fresh water mud intrusion and saline mud invasion.(2)Study on the mechanism of mud invasionDetailed analyses are made to study Ytong basin mud invasion mechanism and the process of mud fluid invading the reservoir、the loss process of the borehole fluid when well drilling、the displacement effect and the profile of the fluid in the invaded zone、vertical activities of the invading mud filtrate and disappearances in the invaded zone after well completion. So brought the following perspectives into being:During the time when the borehole comes into being, mud and mud filtrate begins to permeate into the porosity layer. And the whole time, different processes have appeared in different time:instant water loss processes before mud-cake taking shape; dynamic filtration process after the mud-cake taking shape; certain time after drilling, static filtration process during trips which stops the mud fluid cycling and the alternative appearances of the dynamic water loss process and static water loss process after that;finally the mud cake keeps a certain thickness, accumulated fluid loss reaches to a certain amount and the dynamic balance process has formed.The process of mud filtration invading the oil or gas reservoir is similar to the process of displacement of oil by water in reservoir waterflood development, but they are different in some way. The invading effect will alter the radial direction of the reservoir resistivity, in the meanwhile it forms a circular mud cake in the wall of hole and flush zone、invaded zone and original layer along the radial of the layer. The process drilling mud invading the reservoirs can be divided into injection, filter and dynamic balance processes. Mud filtrate and formation fluid mainly have three physical processes:displacement, mixed, diffusion. The displacement effect of the fluid in hole of the invaded zone and its distribution depend on mud type, the nature of the formation fluid, formation porosity structure and wettability of the particles, etc. After casing and well cementing, the hydrostatic pressure in borehole disappears immediately, mud radial intrusive action stops completely.(3) Analysis of the influencing factors of the mud invasionCombined with the actual situation in YTong basin, we made detailed analysis of the influencing factors of mud intrusion, including:mud cake porosity, mud cake permeability, mud column pressure, reservoir mud soak time, fluid property, formation porosity, formation permeability effect and so on. We get the following ideas:The degree of mud invasion depends on the factors such as the positive differential pressure(Δp) between drilling mud column pressure and original formation pressure, mud properties, formation characteristics, oil-water phase permeability, diffusion and convection between different salinities water and soaking time, among which the most important factor is the Δp and the formation physical featuresThe higher the mud column pressure is, the bigger the differential pressure of the invasion is. When objective layer conditions remain unchanged, invasion deepens as differential pressure of invasion increases. If the outer boundaries of the invasion differential pressures are the same, then we can get the following conclusion:invasion depth in the gas layer is the deepest, invasion depth in the oil layer takes the second place, while invasion depth in the water layer is the shallowest. When mud cake permeability becomes decisive factors of the mud filtrate invasion, the greater the porosity is, the smaller the depth of mud filtrate invasion is, and vice versa. When the formation porosity and permeability properties become the decisive factor of the mud filtrate invasion, the better the formation porosity and permeability are, the more the filtrate infiltrated from the mud cake into the formation is, thus the deeper the invasion depth is.(4) Numerical simulation of resistivity under mud invasionProceeding from the basic principles of electrical logging and combining with finite element theory, resistivity during the process of mud invasion formation forward simulation numerical modeling work has completed. The following achievements have made:Calculative K-factors of deep and shallow dual later logging are equal to the K-factors given by manufacturers and the error is less then1%. Production of deep and shallow dual later logging resistivity borehole correction chart and logging (Zhang Gengji) contrast correction chart is in coincidence.Used the method that borehole and invation have been correted at the same time.It has resolved the problem of current focus degree change when borehole correction and invation correction of conventional correction have been treated separately. Based on that, completed numerical simulation and correction chart of deep and shallow dual lateral resistivity in different borehole diameter, different mud, different invasion condition are more in accord with the logging facts.(5) resistivity correction under mud invasionFormed a mud invasion correction method in Ytong basin:Tried to use lateral electrode coefficient to calculat formation resistivity and invade radius.Study on induction resistivity well logging to calculat formation resistivity and invade radius.Proposed equivalent invasion correction model based on step-profile to evaluate formation resistivity. Utilizing optimization to approach dynamic invasion, Step-profile and actual formation situation have been combined. In contrast to step-profile model of conventional correction chart, using dual lateral log date to calculat formation resistivity has better effect in practical application.