Diammonium Phosphate in Oilwell Cement: Fresh-State Evaluation by Thickening Time and Rheology

Phosphate-based additives are of growing interest because of their influence on early hydration and potential durability benefits in aggressive environments such as CO₂-rich conditions. However, their impact on fresh-state behavior under oilwell cementing conditions remains insufficiently understood. This study evaluates the effect of diammonium phosphate (DAP) on the rheological and thickening-time behavior of additive-free API Class G cement slurries tested at atmospheric pressure and 85°C. Cement slurries containing 0.25-1.00% DAP by weight of cement were evaluated using atmospheric thickening-time and rotational viscometer measurements. Mixing method (dry versus wet mixing) and short-term thermal exposure were also examined. DAP altered fresh-state behavior at the investigated dosages. In dry-mixed systems, the time to reach 70 Bearden consistency decreased from 121 min for neat cement to 64 min at 0.50% DAP, accompanied by a steeper stiffening rate and a shortened pumpability window. Rheological measurements showed an increase in yield point (15-36 Pa) and 10 min gel strength (11-27 Pa) with increasing DAP content, while plastic viscosity remained comparatively unchanged, indicating enhanced early structural buildup. For the investigated 0.25% and 0.50% BWOC wet-mix formulations, wet mixing delayed the onset of structuring and reduced conditioning-induced gel development relative to dry mixing. These observations suggest that the DAP response depends on dispersion pathway and early thermal history. Overall, thickening time alone is insufficient to assess placement stability in DAP-modified systems. Combined thickening-time and rheological evaluation helps identify formulations prone to premature structural buildup under the investigated atmospheric-pressure, additive-free API Class G conditions at 85°C.