Method of preventing gas migration during primary well cementing

A method of cementing a string of pipe in a well bore which penetrates a gas formation while preventing gas migration in the well bore, both the pipe and well bore being substantially filled with drilling fluid, comprising the steps of:

• (a) displacing a cement slurry downwardly through said pipe and upwardly into the annulus between said pipe and the walls of said well bore while maintaining sufficient hydrostatic pressure on said gas formation to prevent the entry of pressurized gas therefrom into said well bore;
• (b) determining the initial surface pressure in said pipe after said cement slurry is displaced into said annulus in accordance with step (a);
• (c) displacing additional cement slurry into said annulus as is necessary to make up for losses in said surface pressure due to cement slurry gel strength development and volume reduction and to maintain said surface pressure substantially equal to said initial surface pressure determined in accordance with step (b) thereby preventing the entry of gas from said gas formation into said well bore until said cement slurry in said annulus develops a predetermined gel strength sufficient by itself to prevent the migration of said gas, said predetermined gel strength being determined in accordance with the following relationship:
• SGS=20.8 R(.DELTA..rho.)
• where:
• SGS=static gel strength ( lbs/100 ft.sup.2);
• R=gas bubble radius (inches);
• and
• .DELTA..rho.=difference in density between said cement slurry and said gas;
• and then
• (d) permitting said cement slurry to remain static in said annulus so that it develops additional gel strength and sets into a hard impermeable mass therein.

A method of cementing a string of pipe having a float shoe at the bottom end thereof in a well bore which penetrates a gas formation while preventing gas migration in the well bore, both the pipe and well bore being substantially filled with drilling fluid, comprising the steps of:

• (a) pumping a fluid separating plug followed by a cement slurry into said pipe;
• (b) displacing said cement slurry with a second fluid separating plug and drilling fluid downwardly through said pipe, said first fluid separating plug including a passageway therethrough which opens when said plug lands on said float shoe thereby allowing said cement slurry to flow into said annulus;
• (c) continuing the displacement of said cement slurry upwardly into the annulus between said pipe and the walls of said well bore while maintaining sufficient hydrostatic pressure on said gas formation to prevent the entry of gas therefrom into said well bore;
• (d) stopping said displacement before said second fluid separating plug reaches said bottom fluid separating plug adjacent said float shoe;
• (e) determining the initial surface pressure in said pipe after said cement slurry is displaced into said annulus in accordance with step (c);
• (f) displacing additional cement slurry into said annulus as is necessary to make up for losses in said surface pressure due to cement slurry gel strength development and volume reduction and to maintain said surface pressure substantially equal to said initial surface pressure determined in accordance with step (e) thereby preventing the entry of gas from said gas formation into said well bore until said cement slurry in said annulus develops a predetermined gel strength sufficient by itself to prevent the migration of said gas, said predetermined gel strength being determined in accordance with the following relationship:
• SGS=20.8 R(.DELTA..rho.)
• where:
• SGS=static gel strength (lbs/100 ft.sup.2);
• R=gas bubble radius (inches);
• and
• .DELTA..rho.=difference in density between said cement slurry and said gas;
• and then
• (g) permitting said cement slurry to remain static in said annulus so that it develops additional gel strength and sets into a hard impermeable mass therein.