Cementing: Primary and Remedial Well Cementing Operations

What Is Cementing in Oil and Gas?

Cementing is the well construction operation that pumps a precisely engineered cement slurry down the inside of a casing string and back up the annular space between the casing and the borehole wall to achieve zonal isolation, provide mechanical support to the casing, protect fresh-water aquifers, and establish a pressure-containing barrier that persists for the life of the well, performed at each casing string set during the drilling program.

How Cementing Operations Work

A primary cementing operation follows a structured sequence beginning with conditioning the wellbore. Before running casing, the drilling crew circulates drilling fluid for a full bottoms-up cycle to remove cuttings and condition the mud. After the casing string is run to the planned setting depth, a centralizer program distributes the casing in the center of the borehole to ensure uniform annular clearance for cement placement. The cementing crew then rigs up the cement unit on the surface, which consists of a high-pressure mixing head, blending tub, bulk cement storage, water supply, and mixing pump capable of delivering slurry at rates between 1 and 15 barrels per minute (bbl/min) depending on annular capacity and pump pressure limits.

The displacement sequence begins with pumping a chemical wash or water-based spacer fluid to break down the drilling mud filter cake on the borehole wall and reduce mud contamination of the leading edge of the slurry. A pre-flush of thinned mud or fresh water often precedes the spacer. After the spacer, the bottom cementing plug is released into the casing. This hollow rubber plug travels down the casing ahead of the cement slurry, wiping the mud from the casing interior wall. When the bottom plug reaches the float shoe, it ruptures at a differential pressure of approximately 300 to 500 psi (2.1 to 3.4 MPa), allowing the cement slurry to flow through and out the casing shoe into the annulus. The cement volume pumped is calculated to fill the annulus from the shoe to the planned top of cement (TOC). After the full slurry volume is pumped, the top plug is released. This solid rubber plug separates the trailing edge of the cement from the displacement fluid and travels down the casing, pushing all the remaining cement out. When the top plug lands and bumps against the float collar, a pressure increase of 200 to 500 psi (1.4 to 3.4 MPa) above circulating pressure confirms plug landing and the end of displacement. The float valves in the float shoe and float collar prevent the cement slurry from flowing back inside the casing under U-tube pressure from the annulus.

After displacement is complete the well enters the wait-on-cement (WOC) period. During this time the crew monitors casing pressure for any increase that could indicate gas migration through the gelling cement. Most regulatory authorities require a minimum WOC period before any additional wellbore operations proceed, and drilling ahead through the cemented casing shoe typically requires confirmation that the cement has achieved a minimum compressive strength of 500 psi (3.4 MPa). After WOC, the driller tags the top of the cemented float equipment with the drill bit, confirms the cement is hard, and drills out the float shoe, float collar, and the cemented portion of the lower casing before resuming drilling to the next casing point.

Cementing Across International Jurisdictions

In Alberta, Canada, AER Directive 009 mandates that all surface casing strings be cemented to surface and that the cement return to surface be confirmed by cement returns at the casing head or by evaluation log. Operators must submit a detailed cement program including slurry design, additive concentrations, calculated volumes, and displacement schedules as part of the well license application. The Directive specifies minimum compressive strength of 3,500 kPa (approximately 508 psi) within 24 hours and requires that operators retain lab test records for the slurry designs used. Directive 009 also requires that operators use API Class G or H cement or demonstrate equivalent performance for non-API blends.

Types of Cementing Operations

Primary cementing encompasses every planned cementing job performed as part of normal well construction. This includes conductor cementing, surface casing cementing, intermediate casing cementing, production casing cementing, and liner cementing. Each operation is designed independently because the depth, temperature, pressure, annular geometry, and operational objectives differ at each casing point. Conductor cementing, the shallowest and simplest job, typically uses a neat cement slurry with minimal additives because temperatures and pressures are low. Surface casing cementing must achieve cement returns to the surface and therefore requires accurate volume calculations accounting for wellbore irregularities measured by a caliper log. Production casing cementing requires the most complex slurry design because the cement must isolate the reservoir from other zones, resist the pressures and temperatures of hydraulic fracturing if unconventional completion methods are used, and maintain integrity throughout the well's productive and subsequent abandonment life.

Liner cementing presents particular challenges because the liner top does not extend to surface and therefore cementing must be performed through the liner running tool with limited ability to verify returns. A liner hanger with a packer seals the liner-to-casing annulus at the liner top, and a stinger or inner string carries the cement to the liner shoe. Liner top squeeze jobs are routinely performed after primary liner cementing to ensure a hydraulic seal at the liner hanger because the primary job's top of cement frequently falls short of covering the liner top adequately. Multistage cementing tools allow operators to cement long casing strings in two or more stages, opening a port at an intermediate depth to pump a second cement stage without exceeding the formation fracture pressure that would result from lifting a single continuous slurry column to surface.

Squeeze cementing (remedial cementing) is performed when primary cementing is evaluated as inadequate or when well integrity is compromised during production. A balanced plug squeeze delivers a cement slurry through a packer and applies pressure to dehydrate the slurry against the formation or through perforations into the void in the annular cement. Hesitation squeeze technique applies pressure in small increments with waiting periods between each application to allow the cement to dehydrate incrementally and set incrementally, building a dense filter cake rather than fracturing the formation. Running squeeze involves continuously pumping while maintaining pressure and is used when the void to be filled is large and formation permeability is moderate to high. Block squeeze isolates an entire interval between two packers for full-circumference repair.

Top jobs are short cementing operations performed from surface to fill the annulus at the top of the casing string when primary cementing failed to return cement to surface. The cement is bullheaded down the outside of the casing through a side-valve or tremie pipe rather than through the casing interior. Top jobs are cost-effective for shallow surface casing strings but cannot address isolation failures at depth.

Cement Job Design and Engineering

Successful cement job design begins with accurate wellbore data. Openhole caliper logs provide the actual borehole diameter profile versus depth, which is essential for calculating true annular volume. A borehole that is washed out to 150 percent of bit size in a reactive shale interval can double the cement volume required for that interval. The cement engineer calculates planned annular volumes by integrating the cross-sectional area over the depth interval, accounting for pipe displacement and inside casing fluid volumes.

Slurry design requires laboratory testing at simulated downhole conditions. The primary tests performed on every cement design include thickening time testing in a pressurized consistometer at the estimated bottomhole circulating temperature (BHCT), free water content measurement (limit is typically less than 0.2 mL per 250 mL of slurry), fluid loss measurement using an API fluid loss cell at 1,000 psi (6.9 MPa) differential pressure (target typically less than 50 mL/30 min for production strings), compressive strength development testing at simulated bottomhole temperature using non-destructive sonic testing on cured samples, and slurry density verification using a mud balance.

Displacement efficiency, the percentage of mud removed from the annulus and replaced with cement, is the most important predictor of cement job success. Laboratory and field studies demonstrate that turbulent flow conditions in the annulus provide the most effective mud displacement, with displacement efficiency exceeding 90 percent at fully turbulent Reynolds numbers compared to 50 to 60 percent for laminar flow conditions. However, achieving turbulent flow in the annulus of a cemented casing string requires pump rates and pressures that may exceed the fracture pressure of weak or depleted formations. In those cases, operators use high-viscosity spacer pills engineered to remove mud channels mechanically, and pump at the maximum rate that stays below fracture gradient.

Cementing Synonyms and Related Terminology

• Primary cementing: the planned cementing operation performed during well construction at each casing string
• Cement job: informal term for any cementing operation
• Squeeze cementing: remedial cementing operation to repair or supplement a primary cement job
• Secondary cementing: alternative term for remedial or squeeze cementing
• Top job: shallow remedial cementing through the casing-casing annulus from surface
• Liner cementing: primary cementing of a liner string that does not extend to surface
• Cement placement: the displacement phase of a cementing operation

Related terms: cement, cement bond log, casing, casing string, surface casing, mud weight, well control, blowout preventer, cementing plug, float shoe, float collar