Huff-and-puff is a key technology for the efficient recovery of oil and gas from tight reservoirs. Active water and CO₂ are two huff-and-puff media with great development potential; however, their effects on enhanced oil recovery and the contribution of imbibition displacement to enhanced oil recovery need further investigation. In this paper, short cores were spliced into long cores for huff-and-puff experiments, and then nuclear magnetic resonance testing was performed to test the transverse relaxation time spectrum of different core sections at different huff-and-puff cycles. Subsequently, the enhanced oil recovery effects, limited effective distances, and influencing factors of active water and CO₂ huff-and-puff were evaluated. Meanwhile, a comparative experiment without well soaking in some specific huff-and-puff cycles was designed to quantitatively split the contribution rate of elastic displacement and imbibition displacement. The results show that active water huff-and-puff mainly mobilizes crude oil in large pores, while CO₂ huff-and-puff can also mobilize crude oil in small pores. The cumulative oil recovery of active water and CO₂ after 4 cycles of huff-and-puff was 24.78% and 40.89%, respectively, and the limited effective distances were 6-8 cm and 8-10 cm, respectively. Elastic displacement is considered the main enhanced oil recovery mechanism of active water and CO₂ huff-and-puff, while imbibition displacement accounts for 20.86% and 31.52%, respectively. Due to its good diffusion and mass transfer ability, CO₂ can more fully participate in the mechanism of imbibition displacement and further improve oil recovery. The findings of this paper can provide valuable theoretical and field data support for the application of huff-and-puff technology in tight reservoirs.

The traditional multi-process to enhance tight oil recovery based on fracturing and huff-n-puff has obvious deficiencies, such as low recovery efficiency, rapid production decline, high cost, and complexity, etc. Therefore, a new technology, the so-called fracturing-oil expulsion integration, which does not need flowback after fracturing while making full use of the fracturing energy and gel breaking fluids, are needed to enable efficient exploitation of tight oil. A novel triple-responsive smart fluid based on “pseudo-Gemini” zwitterionic viscoelastic surfactant (VES) consisting of N-erucylamidopropyl-N,N-dimethyl-3-ammonio-2-hydroxy-1-propane-sulfonate (EHSB), N,N,N′, N′-tetramethyl-1,3-propanediamine (TMEDA) and sodium p-toluenesulfonate (NaPts), is developed. Then, the rheology of smart fluid is systematically studied at varying conditions (CO₂, temperature and pressure). Moreover, the mechanism of triple-response is discussed in detail. Finally, a series of fracturing and spontaneous imbibition performances are systematically investigated. The smart fluid shows excellent CO₂-, thermal-, and pressure-triple responsive behavior. It can meet the technical requirement of tight oil fracturing construction at 140 ℃ in the presence of 3.5 MPa CO₂. The gel breaking fluid shows excellent spontaneous imbibition oil expulsion (~40%), salt resistance (1.2 × 10⁴ mg/L Na⁺), temperature resistance (140 ℃) and aging stability (30 days).